Magical Jelly Bean Keyfinder is an open source freeware utility that examines your Windows registry and returns the Product Key for Windows or MS Office.
Great for making a backup of the key if you have lost the original written copy.
It works on Windows 95, 98, ME, 2000, XP, Vista, Server 2003, Server 2008, Office XP, Office 2003, and Office 2007.