What you need to do is put in your XP disk, reboot, and press F12 or whatever your boot menu is. then press boot from ide-cdrom or something that has to do with booting form your cd drive. then it will load a bunch of files and you will come to a blue screen. press install fresh copy of windows. then it will bring you to a menu with all your partitions. highlight the partitions and delete them all. then go and press create new partition. after you have done that press install windows onto selected partition with the new partition highlighted. It will start to format and form there its just installing windows.
please follow ALL steps.
i have done it 12 times and it has worked every time.