Find Windows keycode from DOS

We get a lot of old hard drives which we want to wipe out and reinstall. Windows is usually to far gone to try and fix, especially when we’re putting the drive on a completely different system. So restoring a backed up registry won’t help.
What we’d like to do is somehow read the registry from a DOS prompt off a boot disk and find the “Product Key” before we wipe out the hard drive, so we can reuse it again. There’s no other way to get the key code when we’re talking about five year old drives that are 1 and 2 gigs each.
The system.dat and user.dat files are still accesable, so maybe we need a binary file reader or something. Any suggestions? Maybe we could use Debug or something.