It's in the registry: c:\windows\system.dat. Have a good look (with Notepad, if it isn't the current registry), and you'll find it.
Toni's lengthy post http://reviews.cnet.com/5208-6121-0.html?forumID=45&threadID=25861&messageID=290112&tag= tells some more about it.
Hope this helps.
Kees
I was running WinMe before I formatted. Like other unfortunate souls I seem to have forgotten that I needed that oh so precious Product Key Code. My question is this: If before formatting I copied the HD contents to CDRW is there any place I can look to find the Key code? I had to install Win98 so I could at least get here. Any thoughts or help is welcomed. Thanks in Advance!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic