1) Get WinXP CD
2) Insert in CD drive
3) Restart PC
4) Enter BIOS, set to boot from CD-ROM first
5) When XP setup asks what to do, delete all partitions on the disk, make new one(s)
6) Install Windows XP
A little bit more (or less, depends how you look at it..) constructive:
Look for a "mygames" folder somewhere in the user profile ... I think I remember reading something about Vista silently moving "save folders" there, somewhere next to "my documents" ..