Is there a way to recover an old Vista product key from a 'windows.old' file?

I've an ancient Dell laptop that I use for church purposes that had to be reformatted recently. Only, given its age the key sticker has fallen off and I'd lost its CD. Terms being such a pal of mine that dabbles in computers reformatted the drive and reinstalled Vista for me. Only, I now think he may have not actually used the proper key. But, one that just happens to work with the motherboard (another techie friend says so, anyway). But, not the unit overall. The old windows is saved. But, I'd like to see if I could somehow retrieve the old key to compare it to the one he wrote down to make sure. These two friends it should be noted don't get along and I may just be between their sparring on the matter.

Thanks.