XP repair requires product key?!?!

I did a repair on one of my Compaqs that had an OEM version of XP Home on it. I used an XP Home Upgrade disc. Once the repair was done it prompted me for the product key - so I type in the key that's on the little sticker on the side of my computer, and it doesn't accept it - it says invalid key!!!

I'm shocked it even prompts for the key in the first place since I'm doing a repair, not a full install...but anyways...any clue as to why it won't work with the code I have? I called both MS and Compaq, and I always get someone that speaks broken english and who doesn't seem to have any clue as to what is going on...and they keep blaming each other :/

Thanks for your time and information.

Did you repair it with a service pack 1 or 2 version when the origianl install was different. That would explain it. If so the key will not work properly. Either use the key from the cd you ran the repair from or call someone at MS. I know they are a pain in the butt.

the XP Home Upgrade serial is different than a regular XP Home serial. That's why it won't accept it.