I think coxchris is correct. It's not necessary to use a totally identical mainboard to enable a Windows installation to remain activated, or to be re-activated.
Microsoft understands that computers may need repair. Where the OEM license is tied to original hardware, it's satisfactory for the replacement to be an equivalent board, so that the system is not sufficiently "upgraded" to become a different computer.
The need for Windows to be re-activated is triggered by a threshold of equipment change occurring within a 120 day window. If the threshold is not exceeded by your change, Windows may not need to be re-activated at all. Also, since the components presumably are electronically the same (same chipset etc) there may be no need to update drivers or repair the install either.
XP does have a lowish threshold, so if the change is sufficient to trigger activation, see if on-line activation is accepted. If not, contact Microsoft and explain that a failed mainboard has been replaced with the manufacturer's nearest equivalent. They should have no hesitation in issuing you with a new activation code.
The link below explains Microsoft's view of activation and the procedures involved - it's intended to be a barrier to illicit software sharing, which is why it's part of a discussion of piracy. You're not doing any piracy, so I don't expect they will have any problem if your new mainboard makes Windows need activation.