I always thought you needed a new license for a new MB w/ XP's stupid licensing, so I dug for a bit out of curiosity...this should be enough info to help out:
http://www.michaelstevenstech.com/moving_xp.html
Ideally, you'd swap w/ an identical motherboard, but if not, only do the Repair option. Otherwise, you need a new activation key. That's how I read it at least...