This is one of those "If you have to ask, you probably shouldn't try it" situations. Just cause it oughta be simple doesn't mean it will be.
Have you checked with the mfr to find out whether XP has drivers for all the hardware?
If the computer is new enough to be supported in Vista you might think, "Of course XP drivers exist," but I had a call last week from someone who backrevved a notebook from Vista to XP & lost sound, LAN connectivity (both wired & wireless), & fell back to base video (generic VGA). Of course, without Ethernet, she couldn't reach the mfr's web site to d/l XP drivers, if there are any. So even if the mfr sez XP drivers exist, you might want to d/l & burn 'em to a CD before starting the backrev.
Also, if your XP key has been used to validate a previous install, it won't work on this computer - or be prepared to phone MicroSnot.
2000 GT: Buncha stuff, more in the works