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.

Do you know of a free or inexpensive program that will overwrite hard drives? My husband and I have updated to new computers, leaving us with two hard drives that have sensitive data that we would like to erase before disposing of them.

There’s a $20 program for Windows called Window Washer from Webroot that will wipe an entire hard disk so that its data is unrecoverable. Its maker calls this process “bleaching.” In cases where you don’t want to completely wipe a hard disk, the program can also securely erase individual files and folders, and can remove all traces of browser activity. I have tested it, and I can recommend it. You can get it at Webroot.com.

There are numerous other Windows file wipers, some of which may be free, though I haven’t tested them. To find these others, go to download.com and enter “file wiper” in the search box.

Since downloading the new Firefox browser, the panel at the top that shows the Web address and the back and forward buttons has disappeared. How do I get it back?

That panel is called the Navigation Bar and it can be turned on and off. It sounds like it somehow got turned off on your computer. To switch it on again, go to the “View” menu, select “Toolbars” and then click on “Navigation Bar.” The missing panel should reappear, and the next time you view that menu option, you will notice a check mark next to “Navigation Bar,” indicating it is on.

You can also use the same technique to switch the Bookmarks Toolbar on and off.

I have switched to Mac recently and never want to use Windows again. But the program I miss using on Windows the most is Microsoft Money. Do you know any way to make this program run on my Mac?

Microsoft (MSFT) doesn’t make a Mac version of Money.

However, all current Macs can run Windows and Windows programs. I suggest you pick up a copy of one of two programs — Parallels Desktop or VMWare Fusion. Both allow you to run Windows programs right alongside your Mac programs, even without displaying the Windows desktop. Microsoft Money would just appear in its own window, as if it were another Mac program.

This method works well, but it carries a price. Fusion and Parallels cost around $60-$70 each, plus you will have to buy and install a full, fresh, boxed copy of Windows.

• 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.