I have a laptop that I use primarily to play music. Is there any way to disable its connection to a wireless network?

Sure, but exactly how you do that depends on the laptop’s hardware and operating system. Some Windows laptops have a hardware switch, or a keyboard function — often identified by some sort of transmitter icon — that turns the Wi-Fi wireless radio inside off. If yours does, just use these hardware controls to kill Wi-Fi.

Others require disabling Wi-Fi via software. In Windows XP, go to the Network Connections control panel, find the Wi-Fi connection, right-click on that icon, and select “Disable.” In Windows Vista, go to the Network and Sharing Center control panel, locate the wireless network, and click the “Disconnect” command in the information area under its name. On a Mac laptop, you can just click on the fan-like Wi-Fi symbol in the menu bar and select “Turn Airport Off” (”Airport” is Apple’s term for its Wi-Fi radio).

If your Windows laptop uses an application other than control panels to manage your Wi-Fi connections, you may have to disable them from within that program.

Thanks for your column on netbooks. Which of the netbooks came pre-installed with Microsoft Office? For those that didn’t, how can you install Microsoft Office onto the device?

I tested only four models, all running Windows XP. None came with a full, activated version of Office installed. Two — the Acer Aspire One and the MSI Wind U100 — came with trial versions of Office pre-installed. Another, the Asus Eee 1000H, came with an alternative office suite, StarOffice. Two, the Asus and the MSI, came with a stripped-down office suite, Microsoft Works.

The simplest way to install Office, or any other software that comes on disc, onto a netbook is to plug in an external DVD drive using a USB port.

I have two questions about the iPhone. My first question is whether or not it’s possible to tether the phone to a computer for use as a modem. My second question is whether it’s possible to rent or buy movies on the phone itself or if that needs to be done from a laptop or desktop?

It isn’t currently possible to use the iPhone as a tethered modem, but AT&T recently said that functionality might be coming. It also isn’t possible to buy or rent movies directly on the phone, only music. You have to transfer movies from a computer.

• 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.