If the WinXP is retail-boxed, as opposed to OEM, you're entitled to move it to a different computer. But you still may need to call in and activate it since the previous activation was on different hardware. I've had to activate over the phone after messing around with too much stuff on my system (temporarily running it without its DVD drive appears to have been the final straw

) and it takes about 4-5 minutes.
If you're not familiar with the activation procedure, it goes like this:
First you read a long string of numerals to a computerized lady who makes annoying encouraging comments, and then she says OMG UR CODE DOES NOT MATCH><0R!!!1!! :Q and puts you through to a polite person on a different continent who asks you if the license is in use on any other computers. You tell them "Bah, only four or five. Maybe seven at the very most.

Or actually, if you really want the truth, just this one." and then they read you off a new string of numerals that you type into the activation box.
Annoying, but not the end of the world. People who don't like this routine can consider exercising their downgrade rights and using Win2000 Pro in its place.