I've just discovered an interesting tool called Keyfinder, now out with the new version 1.5 beta 3.
What is this tool? This is a freeware utility that retrieves your Product Key (cd key) used to install windows from your registry. It can even view remote keys, if you have remote registry rights, and it can tell keys of Office products too.
The tool uses the method described in HOW TO: Change the Volume Licensing Product Key on a Windows XP SP1-Based Computer (328874).
I've not tryed it... seems good.