The cd-key is probably encrypted when it goes into the registry, just for the reason that you want to now:
So that people can't get it out of the registry. Poke around a little more and see if that really is the key... it might have been stored somewhere else. Even look in your BF2 directory for a file called "cdkey.id" or something. I know Doom 3 has one of those... maybe they did it for BF2 too?