From OTT Solutions:
Product Key Finder is a free tool that will display the installed Windows Product Key and other important Windows system information. Product Key Finder works with the following Windows operating systems (2000, 2003, XP, Vista, 2008, 7), 32-bit and 64-bit. You will be able to save all product keys to a text or an xml file.
Product Key Finder is great if you ever need to find out your product key for Microsoft Windows, Microsoft Office and a number of other products. Windows does not provide a way to view the installed product key. If you have lost your key or the registration card that it is printed on, you are out of luck. Re-installing Windows requires you to have the original product key. You are even able to change your Windows product key instantly without having to re-install.