If you’ve lost your Window License Key, and it’s not on the side of your machine, you can retrieve your key by using a little application called Keyfinder, by Magical Jelly Bean. You probably want to grab the 2.0 beta, as 1.5 is a little old and doesn’t support the latest operating systems.
In addition to retrieving keys for Windows 95, 98, ME, 2000, XP, Server 2003, and Vista; Keyfinder also finds keys for Office XP, Office 2003, and Office 2007.
For Small Business IT guys, as an added bonus, you can easily change your Windows key from the Tools menu. Handy if you’ve accidentally installed two PCs with the same license and need to change one (without having to re-install).