you can try to use EasyBCD live search it ... and then use it remove windows boot partition for vista ... after that boot in xp and format the drive with vista.. you might get through else you will get a error message ...
Insert the XP CD, boot from it.. press repair.. follow the onscreen instructions and type.. fixboot at the end and press exter.. just till here and ask whats unclear after that... just do till booting up
