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.

I need to use Microsoft Publisher documents on my MacBook. Unfortunately, I can’t figure out how to do this. I have tried to run it on Microsoft Word, but this didn’t work.

As far as I know, neither Microsoft nor Apple nor anyone else makes a native Mac program that can open or edit Microsoft Publisher documents, which use their own special format, rather than any common cross-platform format.

However, there may be some workarounds. If you own or can borrow a PC and a copy of Microsoft Publisher, you could export the documents as PDF files from within Publisher, and then open them on a Mac. Or you could run Publisher itself on your MacBook, since Macs can run Windows and Windows programs, if you buy and install Windows.

Your Mossberg Solution column recently recommended an add-on for Firefox called Surf Canyon that improves searches. But I am wondering if it’s safe to use, since it is labeled “beta.”

I have been using Surf Canyon in Firefox for months, without any problems. (It also works in Internet Explorer.) You are correct that it’s a beta, but that label doesn’t usually mean a product is unsafe, just that it’s incomplete or unpolished. In the old days, a “beta” product was usually pretty flaky and available only to a very small number of testers until the kinks were worked out. Now, especially with Web-related products, a “beta” is often just another word for version 1.0 of a product. It is open to all, and may not work perfectly, but is usually not dangerous to your computer.

Is there a legal way to use a smartphone as a modem for a laptop?

Yes, indeed. It depends on your carrier, and your plan, and your phone, but it can be done, perfectly legally, either using a cable or Bluetooth wireless to connect the phone to the laptop. Note that, depending on your plan, the carrier may well charge you an added monthly fee for this privilege. Also, the speed of your connection may be a bit slower than the speed you’d get from a cellular data card you insert directly into the laptop or from one that’s built in.

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

Should I install Microsoft’s “SP3″ upgrade to Windows XP? I hear opinions on both sides.

In general, I recommend this upgrade, officially called Service Pack 3, because it beefs up security, and that helps not only you, but all Windows users, to avoid the spread of malicious software. If your PC is more secure, it is less likely to be used as a source from which other PCs can be infected.

I have upgraded a couple of XP PCs to SP3 without any downside. However, like you, I have also heard from some users that the upgrade caused them problems.

As with any major operating-system upgrade, either outcome can occur, depending upon your computer’s particular configuration and condition. So, if you’re on the fence, consider your level of concern about security.

How do cellular data cards compare with fast cable modems in terms of Internet speed? Are these cards more secure than Wi-Fi?

In my experience, using and testing cellular data cards on various laptops, they typically deliver, in real day-to-day use, somewhere between 300 kilobits per second and 1.5 megabits per second. By contrast, a fast cable Internet service can actually deliver up to 16 mbps. So cable is much faster.

Your security question is much more complicated. Wi-Fi security can range from very poor to quite good. It depends on where and how you use Wi-Fi, how you or others have set up the wireless network, and how you have configured your own computer.

Accessing the Internet over a cellular network is a more controlled process, because the cellphone-network operator manages the transmission, and in most cases supplies and configures the software on your computer. So, it is likely to be safer than the worst Wi-Fi setups. But I would never suggest that cellular data card transmissions are invulnerable to hackers or criminals.

What’s the difference between file-based backup and image-based backup software? Why would I choose one over the other?

Image-based backup software copies an image of your entire hard-disk, including all the programs, settings and obscure files that you never see. While it can be used to recover individual files, in some cases, it is mainly designed to allow you to restore your entire hard disk in the event that you lose it all.

File-based backup software, which is more common among consumers, is typically designed to preserve a copy of only your personal data, the material you can’t re-purchase or re-install if your hard disk dies.

That includes word-processing files, spreadsheets, presentations, emails, contacts, calendar items, photos and other items you created. It also is often used to back up music and video files that would be time-consuming, or very expensive, to re-acquire. And in some cases, it can preserve settings and preferences, such as Web bookmarks.

Both types of backup generally begin with a time-consuming initial session, followed by shorter sessions to update the backup with incremental changes.

Many people choose file-based backup because it occupies less space, and is generally quicker and also less expensive. Image-based backup is for folks who are willing to spend the added money and time to be sure they can restore their whole computer, or clone its contents completely to another machine.

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

A friend of mine suggested that I “defragment” my computer every once in a while. He says it will boost performance, but he also said it will move my data around. I tend to be very organized as to where I put documents on my computer. Will defragmenting my computer jumble my files and put them into random places?

No. Defragmenting a hard disk doesn’t change or scramble the organization of files and folders that you have established. Your file organization should look just the same after defragmenting the disk. What defragmenting does is rearrange the location on the disk of the data segments that make up your files. In some cases, a single file like a word-processor document might actually be made up of multiple data segments, invisible to you, that are located in widely separated physical spots on the disk. Defragmenting tries to optimize the location of these segments so that the computer operates more efficiently.

I saw your review of Norton Internet Security 2009, and wonder what is the difference between this product and Norton 360?

Norton 360 has the same features as Norton Internet Security, but adds some additional ones, like online backup and PC “tune-up.” It also costs more. However, the current version of Norton 360 doesn’t include the improvements I wrote about in the NIS product that make it faster and less of a burden on your computer. Those are planned for a future version of Norton 360.

Can I use one of the new 3G iPhones as a wireless modem for my Apple laptop?

No. That function isn’t built into the iPhone and Apple yanked from distribution a third-party program that enabled using the phone as a modem. I suspect the reason is a business one, not a technical one. Cellphone carriers like AT&T typically take the position that connecting a laptop to their networks, whether via a data card or via a cellphone used as a modem, should command a higher monthly data charge than they typically levy for a cellphone alone.

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

I have a USB cell phone data card that I use with my laptop. This card offers me world-wide connectivity wherever there is GSM cellphone service available. Is there an adapter that will allow me to use this card with the iPod Touch?

I know of no such adapter, nor of the software that would be needed to make one work. However, although the iPod Touch has no cellphone capability, it does include Wi-Fi, which can give you high-speed Internet access world-wide whenever you are in range of a Wi-Fi wireless network.

What U.S. cellphone service provider should I consider when purchasing a new contract and phone that can be easily used in most western European countries and also get good coverage in the U.S.?

In the United States, the only two major carriers that use the technology compatible with European networks are AT&T and T-Mobile. As to which has better coverage in the U.S., my own anecdotal experience is that this varies greatly depending on where you are, and that neither has coverage that’s as good as Verizon’s in as many major cities. However, Verizon (like Sprint) uses a technology that isn’t compatible with European networks, though it has one or two phone models that include both technologies.

I have just switched to the Mac from Windows. Is there a tutorial somewhere on how to use the Mac user interface and operating system?

There are a couple of books on switching to the Mac, which you can find at online bookstores. Also, Apple maintains two free, illustrated online tutorials called “Mac 101″ and “Switch 101,” the latter being specifically for former Windows users. They are available at apple.com/support/mac101 and apple.com/support/switch101.

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