Keyfinder

The Magical Jelly Bean Keyfinder is a freeware open source utility that retrieves your Product Key (cd key) used to install Windows from your registry. It allows you to print or save your keys for safekeeping. It works on Windows 95, 98, ME, 2000, XP, Vista, Server 2003, Server 2008, Office XP, Office 2003, and Office 2007 family of products. It also has a community-updated configuration file that retrieves product keys for many other applications. Another feature is the ability to retrieve product keys from unbootable Windows installations.

http://magicaljellybean.com/keyfinder/