Some versions of windows had the CDKey in the registry. I think they fixed that in 2000 though.
Basicly, because MS sells you the license, and not the media (well, you get the media too) for Win2k, it's useless without a CDKey. You *might* be able to call up MS and get a new key issued, but that would probably require your old CD and a proof of purchase. There would be info on MS's site about that process, if it exists.