Here's a big secret: You don't really own Windows XP. Even when you buy Windows at the store, or it comes preinstalled on your new computer, it's not yours. No, the fine print says that only Microsoft owns Windows. You only own a license -- permission -- to run Windows on your computer. Worse than that, you're only granted permission to run Windows on a single computer.

In the past, many people bought one version of Windows -- one for both their desktop computer and for their laptop. And why not? They either used their desktop computer or their laptop -- they never used them both at the same time.

Windows XP changes that with its new Activation feature. When you install Windows XP, an annoying window pops up, asking you to "activate" your version of Windows. When you click the Activate button, Windows XP takes a "picture" of your computer's components, links them to the serial number on your copy of Windows XP, and sends that information to Microsoft over the Internet.

Then, if you or anybody else ever tries to install that same version of Windows on a different computer, Windows XP says you're using somebody else's version of Windows XP, and it won't work.

• Okay, what happens if you don't bother to "activate" a copy of Windows XP? It simply stops working after 30 days. The new Activation feature ensures that each copy of Windows XP will only work on a single computer. Even if Windows XP came preinstalled on your new computer, you can't take the bundled Windows XP CD and install it on another computer.
• No Internet connection? Then you must call Microsoft's toll-free number, talk to a customer service representative, and activate your copy of Windows by typing in a 25-number password.
• If you want to install Windows XP on several computers, it might be cheaper to purchase a special multi-version license, called a Microsoft License Pak.
• If you upgrade your computer -- adding lots of new parts -- Windows XP might think it's been installed on a new computer and stop working. The solution? You must call Microsoft's toll-free number and convince those folks that you're not trying to steal their software.