I have the key that came on the original packaging ...
BUT once when I was looking in the Explore file manager, I THINK I noticed that the key was the serial number. So if you have it loaded on another system maybe you can find the serial number and try that.
I've been toying with the idea of combining my leisure PC and work PC into one unit, and if I do that then the system I used for XP will become available allowing me to have some fun. We'll see.