The only two I know of that are bootable is Keyfinder Boot disk.It is not free but has a trial version.I dont know what if any limitations might associated with the trial version.
http://www.product-key.com/keyfinders/keyfinderPlugin.htm
The other one is Windows Key Finder.it is also not free
http://www.windows-key-finder.com/
Both are bootable Keyfinders that I think are what you are looking for as both will recover the Windows product key from a non-bootable system