dual boot machine restart after login

Hi can somebody please help me? I have a client that had xp pro loaded on her machine that was infect with a virus, she then took a xp home cd and started a fresh installation of which she did not have the cd key for. Is there anyway (without) formatting the hard drive to remove or uninstall the Xp home version as i think this is what has corrupted the machine and now reestarts after logging in.

Error message failed to read temp.exe and then shutsdown the pc.

our help in this will highly be appreciated!!!!!!!