I'm not sure if it is what you want, but try this.

Do a Google of "produkey"

I have this program and it tells me the key for a number of Microsoft programs on my Computer.
It gives me the key serial of about 25 characters that was needed whem I originally installed Windows XP.

I'ld attach it to this post, but I don't know if that is possible because it won't accept .exe files.

EDIT: - I just realized it may be no good because if your Operating System won't start, then you can't run the little program.
Do a Google anyway, you may find someone had a similar problem to you.