XP Product Key not working on a Different XP CD

My Friend got an old laptop, it has a XP Product Key on the bottom of it. But he didn't have a XP CD. So I let him use mine CD. But the key didn't work. So we tryed 2 more copies of XP CDs, still no luck. we ended up just installing Windows 7 Beta so he could have a temporary solution. (And he wanted to try it out). Any ways I am getting really annoyed, to have a license key and not be able to use it. Any suggestions?

Oyeah is there a way to know if a key is going to work with a CD with out having to waste 15-20 minuets for the CD to load all of the files. Spending hours putting XP CD in and typing 25 charters only to get "The Key you ended is invalid" got really boring.

Never mind it turns out that the CDs we where trying were upgrade disks. I Forgot they were. So I guess that a XP Home Key wouldn't work on a XP home Upgrade CD.

Do you know any way around this problem.
I know there is a change product key button in XP once it is all installed. Do you think the key would work then. Or is it still using the same key verifying software as it was before