Alright, if windows xp is already installed then all should go well. However if you try to reinstall it with the new motherboard, you'll have to call microsoft and ask for a new key. Make sure you tell them that you got a new motherboard.
I'm not sure but I think you can only renew XP 3-5 times. After that things start to look suspicious. And don't renew XP several times in a short period of time.