Assuming you have backed uip all of your files you do this:
1 - reset your computer and as soon as it powers off repeatedly press del, f2, f8 (one of those buttons will send you to SETUP, impossible to know which one although i think its usually del)
2 - go to the BOOT menu, and change the settings so that the primary BOOT device is the DVD drive
3 - insert your windows XP disc
4 - reset your computer and follow the onscreen instructions!