It depends if its a full version of XP or an OEM version. If its a full version then there won't be a problem. If its OEM then you might have to contact Microsoft and explain that the board blew up and you had to replace it. I'm not sure what would happen then to be honest as I've never tried it. If you replace the motherboard with exactly the same motherboard then I assume that there would be no problem at all, but I'm not certain of this either as to be honest I've never had to do this either.
I'm sure of the more knowledgeable members who have had this problem will advise you.