Ah - You have the Hard Drive in your posession which was taken from a Vista machine, and you want to plug it into your XP box and run it.
I don't think M$$FT Licen$$ing will let you do that. Vista registers itself to the mobo. There might be a way to get around it, but I'm not aware of what that might be.
What you might want to try is asking in the Vista area of http://www.msfn.org
Here nearly everyone is "just" a user. Some of the guys there are pretty serious techs.