Welcome Yatyas!
Personally, I think that the install sequence should begin thus:
1: Backup all non-recoverable data from C: hard-disk. (Documents etc.)
2: Format C: to remove Windows Vista.
3: Install Windows XP.
4: Reinstall programmes and backed-up data.
5: Continue as mentioned above....
Vista sucks life out of all but the most modern computers.
(Note that this post isn't entirely serious! If you DO format your drive, and it all goes wrong, please don't blame me!)