On newer generation computers you may have noticed that the windows key sticker is no longer on the bottom or side of the unit. This can make for some frustration when trying to do a clean rebuild. Well chances are the key is stored int he BIOS of the machine and if you are in windows it is extremely easy to get to. just open a command prompt and enter this command.
wmic path softwarelicensingservice get OA3xOriginalProductKey