Useful tool here
The Magical Jelly Bean Keyfinder is a Windows utility that will display the CD key
of your PC’s currently installed version of Microsoft Windows and Microsoft Office.
If you decide to reinstall from scratch and you’ve lost your CDs or the Windows sticker’s
fallen off the computer case, use the Magical Jelly Bean Finder to back up your license
for possible reinstallations. This utility lets you change your Windows activation
key as well (say, if you’ve installed Windows from an existing disk image and want
to activate the new installation with its own key). Free download, Windows only.