Actually it might be tricky. I have yet to play with vista mostly because I refuse to. But here is what you can do.
If the system is booted into vista and your logged into Administrator, you can try and plug your XP cd in and try to install but chances are you wont be able to downgrade that way. Depending on what file system you installed with (NTFS, WinFS), you can put the XP cd in, restart the computer and do a clean install that way. If you have data you cant save and dont want to lose then just do an install without formatting. That should replace teh Vista OS with XP but the Vista drivers and such will still be there. Its not a big deal, sometimes I find it handy to have drivers laying around ;-)
Or just whipe the OS completely and start from scratch. The whole process should take an hour or so depending on how fast your system is. Any questions, just respond.
josh_rain