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.

I am in the market for a new HDTV and the newspaper ads are using terminology that I’m unfamiliar with. Do TVs rated at “720p” provide the same quality picture as those rated at “1080p”?

Technically, the answer is no, but it may not matter. The 1080p resolution is certainly higher, but almost nobody can tell the difference between the same material shown in the two resolutions on TV screens up to around 50″ in size and at the typical distances from which people watch those screens. Not only that, but most sources of video content, with the exception of Blu-ray discs, can’t even fully utilize 1080p. Major TV networks don’t use it yet because it requires a lot of bandwidth.

If you can afford a set that can handle 1080p, you might want to buy it so that you are ready in case a lot of 1080p content one day becomes available. You might also want a 1080p set if you are a videophile; have an enormous screen or a projector that fills a large wall; or if you play a lot of Blu-ray discs and believe you can discern the difference on a typical-sized screen. Otherwise, you could save money by buying a 720p set and you might never know the difference.

In 2006, you recommended a powerline adapter for Internet access by Netgear, the XE104. Is this still a good buy or are there others by now that are better?

I haven’t tested powerline adapters, the gadgets that route computer networks over standard home electrical wiring, since that date. Netgear and its competitors — such as Linksys and Belkin — have, naturally, come out with newer, faster units since then. But I am still personally using the XE104 successfully and feel I continue to get my money’s worth from it. It is still being sold. The newer units typically have greater speed in order to do a better job of streaming video around a home, but they work in basically the same way.

I have a Windows XP system, and things work well with my cable modem in my office. But when I’m on the road using Wi-Fi, I can receive emails, but can’t reply or send out. Any idea on how to resolve this problem?

This usually happens because the Wi-Fi provider is blocking the outgoing email server (called an “SMTP” server) that you or your IT department has set up in your email program. Some providers block all such outgoing servers. There are a number of possible solutions. The simplest is to use a Web-based email service, like Gmail or Yahoo Mail, or the Web-based version of your usual service. If your email is provided by your company, you may be able to access a version of Microsoft Outlook over the Internet that will work.

Another possibility is to ask the provider at the hotel or airport what SMTP server it does allow — usually its own — and enter it into your email program’s settings, if you know how. Yet another option would be to use a data card from a cellphone carrier, which I have found can usually overcome this problem. There may be other workarounds, and I invite readers to suggest them.

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