One of the Windows utilities is Magical Jellybean Software’s Keyfinder. The latest beta version (Keyfinder 2.0 Beta 2–1/2) works with all versions of Windows XP and Office and with 32–bit versions of Windows Vista. It’s discovered that it even works with VMWare product keys.
Alas, it doesn’t work with Windows Vista 64–bit versions. So how do you track down an installed product key if you’ve gone 64 bit? Use RJL Software’s Windows Product Key Viewer. It’s not as pretty or as comprehensive as Keyfinder, but it works.