xp recovery problem

Helping someone with a gateway 400sd4 laptop running xp pro, trying to cure some config problems and get rid of some malware. System still functional but some components of xp missing/not working, so decided to do a repair from an xp cd. Made full backup of Documents & Settings, copy of Repair folder, and much else. Booted the cd, selected Setup, chose the repair option, got it started and went off to do some other things. Came back a while later (never leave Gates alone while you're not watching his every move) and the machine had rebooted and was completing an upgrade to xp pro. Yes, this was obviously an oem copy of xp on the laptop, and I stupidly used a retail upgrade (xp home to pro) cd for this - but I could swear I've done this before and as long as you're only repairing the system it should have been ok. But my memory is not great so maybe that was a big error on my part. Now the 'upgrade' wants the product key to finish setup. Obviously it won't take the old (oem) key, and I really don't want to use a new one. How can I remove/undo the 'upgrade' and restore the previous install of xp pro? I have the hard drive, I can remove it and mount it on another machine if need be, I have the backup of Docs & Settings, I have the repair folder from before the mess happened. Should I just restore the hives from Repair into /system32/config and restore Docs & Settings? Where else should I look or clean up to get rid of the 'upgrade?' Thoughts and suggestions welcome.