I have run into this before, but haven't really researched it. I am horrible about keeping my Windows XP CD's and I usually keep them as ISO's and burn them to a CD when needed. Sometime a particular key doesn't work with a CD so I try the next CD and it works. Are there different versions or something? I don't think it is the OEM version. I have the regular key on the sticker on the side of the case and it says it's an invalid key.

Thanks!