A quick search of these forums or the search engine of your choice should turn up details on how to do this.
I had a new motherboard installed. I forgot to bring my copy of xp home so the tech used his to do a repair install. It worked so I didn't have to reinstall programs. Magic jellybean shows his cd key is in the registry. When I tried to change it to mine from my copy of xp it said the number was invalid. Is there any way to change the cd key to what's from my copy of xp?