All it does in the upgrade process is search for a qualifying install of the previous...it doesn't matter what drive/PARTITION is chosen to install to as once qualified it could care less...

This can also be accomplished with no OS installed...at the qualifying prompt all that's required is to insert the OS CD...it'll READ it and continue the install.OK, thanks patio. It just seems a bit odd because you can end up with two OS in that case by using an upgrade VERSION of the OS and installing it on a second HD or different partition. Perhaps this is not an issue with Microsoft because the user would only be running one OS at a time.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.