boot from the xp cd and delete all the partitions and create new 1s. you have no option but to do this because they are different on xp to win 7.
you will also have to go into bios and disable hpet mode b4 you install the new o.s
hpet is needed by win 7 and vista but not xp.
hpet is the high precision even timer. it will be either set to enabled,32 or 64 depending on the o.s bit rate you were running 32 for 32bit and so on. make sure its disabled b4 installing xp as it can lead to random bsods with xp and you dont want that happening when your installing a new o.s.