You will have to reactivate XP because of the mobo change, and that is hard to do without the product key. It probably won't boot anyway because the chipset drivers will be different for a different chipset.
I have in the past, and am sure some of these guys here, have done exactly that and had no problems. When you install xp it puts a lot of software on your drive that allows lots of hardware run. If you have the cd that comes with the -motherboard- you might get away with it. Then all you would need to do is a windows update. Now, since Mr Microsoft has changed things with the authentication bs you might not get away with it. It's been a few years since I've done it so I don't know. Worth a try IMO.
Is this a home built original or a dell or gateway or something ?