Yes I wondered that too, but the guy who's computer it is said thats what he had done. When I looked there def was xp on drive 1 and vista on drive 2 and all he had was vista upgrade version so it stumped me too. I did'nt even know it was possible.
However I have since recovered all his data and wiped both drives and then re-set up xp pro for him.
Thanks everyone who took the time to post.