illegal copy xp pro Help needed.

Hi Guys, just trying to pick your brains. My wifes just been given a pc from a friend but has apparantly previously had an illegal copy of xp pro installed. The harddrive has been formatted so at boot up it goes for an xp insall and asks for the disc which we dont have. I do however have a disk for xp home for my laptop. Is there anyway i can reformat the HD on my wifes pc before xp tries to intall so i can install xp home. Can get into bios and a sort of dos menu but cant run in safe mode without xp running. Suggestions welcome as its driving me mad. cheers.