In my experence that going from xp to vista is not always a good move on that old pc.
1) If you have another hard drive thats inside that pc, then backup all ur data that you wish to keep.
2) If ur vista install created a hidden partition then you need to remove that, i think to start a complete fresh is best, so you need to download Darik's Boot and Nuke click here
you can put this onto a pen drive or burn to a cd.
3) Remove any hard drives that you do not want to wipe then put the cd with Darik's Boot and Nuke (Dban) and once it boots up into dban just type "autonuke" and then walk away till it's completed.
Basically what this is doing is removing all from that hard drive so you can start a complete fresh.
4) Put xp cd into cdrom> format to ntfs> install xp.
Best of luck..
P.s, some may think this an overkill, but i know this does a good job.