On two different new computers, I’ve created Word files and they’re automatically saved with the extension “.docx.” What is that? When I send these files via email, no one can open them. Can you shed some light?

In the 2007 version of Microsoft Office for Windows, which you appear to have on your two new PCs, Microsoft introduced new default file formats whose extensions end in the letter “x”. The one for Word is “docx,” for Excel it’s “xlsx” and for PowerPoint it’s “pptx.” You are using Word 2007, and all of your files are thus automatically being saved in the “docx” format.

Your correspondents can’t open these files because they are using older versions of Office that don’t recognize the new formats. Microsoft has made available a free auto-conversion patch for the older Windows Office versions, but most people don’t have this patch. (It can be found at microsoft.com/downloads under “Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats.”)

There’s also a kludgy free stand-alone converter from Microsoft for the Mac versions of Word and PowerPoint, available at microsoft.com/mac under the name “Microsoft Office Open XML File Format Converter for Mac.”

Unless you can persuade all of your correspondents to install these workarounds, I suggest you change a setting in Word 2007 so that, henceforth, all of your files will be saved in the traditional “doc” format. That familiar format isn’t only compatible with older versions of Microsoft Office, but with many other programs, on both Windows and the Macintosh.

To make the change, first click on the round “Office Button” at the top left of Word 2007. Then, at the lower right of the window that appears, click on “Word Options.” In the next screen that comes up, click on “Save” in the column at the left. In the panel that appears at the right, you’ll notice an option called “Save Files in this format,” with a drop-down list of choices next to it. Display the list of choices by clicking on the arrow and select “Word 97-2003 Document (*.doc)”. Then, click OK at the bottom of the window.

Microsoft warns that some new features in Word 2007 won’t translate into the old format, but I believe that this will prove irrelevant in 99% of cases and is outweighed in any case by the incompatibility you have run into.

How can I transfer all my Outlook contacts from a Windows PC to a Macintosh contacts program?

There are a number of methods. You can use a PDA or smart phone that can synchronize the contacts with a Mac, after having first synchronized with Outlook on your PC. Or you can use a $10 program called O2M, which is specifically designed to move Outlook data to a variety of Mac programs. It can be downloaded at littlemachines.com.

Or you could get Apple to do it. The company offers a basic file transfer for free if you buy the Mac from an Apple retail store. But, if you want Apple to move Outlook contacts in just the way you want, you may have to purchase the company’s $99 a year ProCare service, which comes with a more complete transfer service.

I recently replaced an old Windows XP computer that was running very slowly. I want to continue to use it as the computer for our young kids, and I was thinking that if I format the hard drive and reinstall XP (which I bought to upgrade the machine a while back), it will probably improve its performance. Do you agree? And, can I reinstall my copy of XP, when I have already “activated” the software when I originally installed it?

I do agree that the computer will likely run faster after you wipe the hard drive and reinstall Windows. That is the usual outcome. And it should be possible to use it legally, as long as you haven’t made any major changes to the hardware, since Microsoft’s activation system typically allows Windows to be reinstalled on the same machine. If activation fails, you can call Microsoft and explain that it’s the same machine, and the company says it will usually OK activation in such cases.

You can find Mossberg’s Mailbox, and my other columns, online for free at the new All Things Digital Web site, http://walt.allthingsd.com.

Write to Walter S. Mossberg at mossberg@wsj.com

Here are a few questions about computers I’ve received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability. This week my mailbox contained questions about PDAs, Microsoft Office 2003’s compatibility with Vista, and memory upgrades for a MacBook Pro.

My wife has requested that we get her first “PDA” and asked me for my advice on which product to buy. I am unfamiliar with the comparative features and function of BlackBerry and Treo. What would you suggest?

Well, first of all, both of these products are primarily communication devices — they make phone calls and fetch email, in addition to performing the functions of a Personal Digital Assistant. So, if she really just needs a PDA, I’d avoid both of them and get a cheaper Palm PDA that isn’t a phone or email device. The $99 Z22 would do the trick, and, unlike a phone, it requires no monthly fee. However, if she wants a phone-and-email device that is also a very good PDA, the Treo beats the BlackBerry hands down. The latter is first and foremost an email device and now has a decent phone. But its calendar, address book and other PDA functions are weak. The Treo, which evolved from the Palm PDAs, is much better at these tasks, in my opinion, especially for personal, as opposed to corporate, use.

Your columns helped me select a new laptop today that should be able to run Vista Ultimate. However, although two of your columns report that Microsoft Office 2003 should run on Vista, two salesmen at Dell said it wouldn’t. Who’s correct?

A: I am, and it’s sad and troubling that Dell personnel would give out such patently wrong information. One might even suspect they were trying to sell you the newer version of Office when you don’t need it. I have personally tested Office 2003 on a Vista computer, and it runs fine. But, just to make sure, I forwarded your question to a senior person at Microsoft and he replied that the Dell people you spoke to are: “misinformed at the least.” He confirmed that Vista supports both Office XP and Office 2003 (as long as they have been kept up to date with the latest service packs) as well as the new Office 2007. One possible explanation is that the Dell folks you spoke to misunderstood your question. Dell and other companies have ceased selling Office 2003, or will soon stop doing so. Therefore, it’s possible that they were under the impression that you were seeking to buy it, rather than merely to install and run a copy you already owned. But Office 2003 is compatible with Vista.

I own an Apple MacBook Pro laptop that came with 512 megabytes of memory. I am going to upgrade to either one gigabyte or two gigabytes. I am not a gamer. Do I really need two gigabytes for running applications or will one gigabyte suffice? The biggest application I use is Final Cut Studio.

In general, one gigabyte is all a MacBook Pro needs for doing common tasks such as email and Web browsing, word processing, music playback and simple photo and video editing with built-in programs like iPhoto and iMovie. However, Final Cut Studio is a massive video-production program meant for professionals, and it demands lots of memory. In fact, Apple’s Web site for the latest version of the product suggests between two gigabytes and four gigabytes, depending on the content you expect to edit and produce. So I would go with two gigabytes, or even more if your model of the MacBook Pro can handle it. In fact, if you are more of an amateur, I’d consider the lighter-duty Final Cut Express, which Apple says does fine with one gigabyte and can handle high-definition content with two gigabytes.

Write to Walter S. Mossberg at mossberg@wsj.com

Here are a few questions about computers I’ve received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability. This week my mailbox contained questions about sending large files over the Internet, buying a combo PDA/cellphone, and sharing music on the same local area network.

Did you review a tool that would make it easier to send large files over the Internet? What was it?

Yes. It’s called Pando, and is a free download at www.pando.com. But there’s an even simpler service, which works right from within a Web browser and doesn’t require the download or installation of any software. It’s called YouSendIt and is at yousendit.com. Both work on the same general principle: Instead of emailing a huge file, you upload it to these services. They then send an email to your intended recipient that merely contains a link or tiny special attachment. When the recipient clicks on the link or special attachment, the file is downloaded to his or her computer, without having passed through his or her email inbox. Both services have free versions, and also paid versions which offer greater capacity and other features.

My Handspring Visor has expired. Since I have also have a vintage Nokia cellphone, would you recommend a combo PDA/cellphone with “useable” keyboard or should I buy a distinct replacement PDA and cellphone?

It all depends on your budget, and your tolerance for carrying two devices. But the stand-alone PDA is fading, and is harder and harder to find. If you bought a Palm Treo phone that uses the Palm operating system, such as the $199 Treo 680, you’d have a decent phone with a built-in PDA that works very much like your Visor, only with more speed and added features. On the other hand, if you’d like a change, there are many Windows Mobile phone/PDA combo devices that can cost even less, like the Samsung Blackjack.

If you’re content to keep the Nokia for phone calls, Palm still makes some standalone PDAs which work pretty much like your Visor, only better — like the $99 Z22.

I read your article on using iTunes to share music with others on the same local area network. Must both computers be turned on in order for this to work?

Yes, all the computers whose music you want to share must not only be turned on, but iTunes must be running on them.

Write to Walter S. Mossberg at mossberg@wsj.com

Here are a few questions about computers I’ve received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability. This week my mailbox contained questions about service to convert home videos to DVDs, portable music players and cellphones with PDA features.

If you have a question, send it to me at mossberg@wsj.com, and I may select it to be answered here in Mossberg’s Mailbox.

Are there services that will take home video and burn it to a DVD that can be played anywhere? I know I can do this on my PC, but it takes too much time and I keep running into problems when I try it.

There are such services. One that I have tested and found to be good is called YesVideo (yesvideo.com). You bring your videos into a store that works with YesVideo — including CVS, Walgreen, Best Buy and Target — and they send the tapes to YesVideo, which converts them to a very nice DVD. You also can get the same service online, at Sony’s ImageStation site (www.imagestation.com). Sony calls its service Video2DVD, but it really is just the YesVideo service. My full review of the service is at: ptech.wsj.com/archive/solution-20040128.html.

Because YesVideo works through retailers, prices vary, but are usually around $25-$35 for a two-hour video. Each DVD is divided into chapters based on a YesVideo process that tries to detect scene changes in your videos. At the end, there are three 60-second music videos made from scenes on your videos. The company also will put your prints, slides and even old film onto DVD, but this costs more and is handled by fewer retailers. Details are at the YesVideo Web site.

I’ve been looking at the portable digital music players, but I’m not sure if any players would work with the media I would download, mainly free recordings of radio programs. Most of the files that interest me are in the Windows Media or Real audio formats, but I usually just see “MP3″ as the type of file the digital music players employ. Are there portable players that handle these formats?

Yes, indeed. All but one of the major portable digital music players on the market can play Windows Media audio files (known as WMA), as well as MP3 files. The exception, unfortunately, is the best and most popular line of players, Apple’s iPod. But you can choose from numerous players made by Creative, iRiver, Rio, Samsung, Dell and many others that play WMA files natively. There are far fewer portable devices that play back the Real audio format natively. Most are personal digital assistants and phones that use the Palm, PocketPC or Nokia operating systems.

Even if you prefer the iPod, you still can use WMA files, with one extra step. Apple’s iTunes software, which manages and transfers music to the iPod, can convert WMA files automatically into MP3 files, a format the iPod can handle — as long as the WMA files aren’t encrypted or copy-protected.

Is there an instrument in the market that is just a phone/PDA, without email and Internet access (which I don’t need)?

Most cellphones have some PDA features, such as a calendar and address book. The more robust “smart” phones, such as Palm’s Treo, the BlackBerry phone models and the phones powered by Microsoft operating systems include a raft of sophisticated PDA features, including the ability to synchronize calendar, contacts and other data with a PC.

However, all of these phones that I have tested also have Internet and email access, which is only natural, because they are communication devices. You certainly don’t need to use the Internet and email features, or pay for a plan that allows you to access them. You can buy a phone with the PDA features you want, and just ignore the email and Internet capabilities.

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Because of the volume of e-mail I receive, I can’t routinely answer individual questions by e-mail, or consult on individual problems or purchasing decisions. I read all questions I receive and select three each week to answer in the column.

Write to Walter S. Mossberg at mossberg@wsj.com