You've got a legal copy, installed and activated. Then your machine goes and blows up on you and you have to install it again. Where's the product key? If your anything like me it probably went out in the rubbish some time ago.
The Magical Jelly Bean Keyfinder is a freeware utility that retrieves your Product Key (cd key) used to install windows from your registry. It has the options to copy the key to clipboard, save it to a text file, or print it for safekeeping. It works on Windows 95, 98, ME, 2000, XP, Server 2003, Windows Vista, Office XP, Office 2003, and Office 2007.
Source, Doug Klippert