Another potential problem:
My friend bought an OEM system builder version of XP Pro from someone on the internet. The laptop has a COA on the bottom, but the product key is very faint, so it's hard to make out.*
Can I install this new copy he bought, or do I have to use the COA that's on the computer?
The situation, I think, was that his original installation of XP failed, and a computer shop installed Vista on it to get it running again. At the bottom of the screen, it says Build 6002, This copy of Windows is not genuine. I think they did this free of charge.
*All of my computers are desktops, but I have their product keys written down elsewhere, in case the stickers get scratched up. Surely, anyone with half a brain would do this for the sticker on the bottom of their LAPTOP! But if I know Rick at all, this will have never occurred to him.