Motherboard replacement, except by the original manufacturer of the computer under warranty is an upgrade that MS considers a "new" computer" and triggers the need to activate Windows again, and unfortunately, if what you have is an OEM version of Windows, according to MS it is not transferable to a "new" computer under the licensing agreement.
You can certainly try to sell your situation to the MS Activation representative but be aware that you may be unsuccessful.
If what you purchased was a "retail" version of XP, it is transferable provided that the previous computer was destroyed or you can convince MS that you removed the operating system from it (which a mb replacement by definition certainly qualifies), and you should have no problem once your activation rep understands what you did.
A reinstall or a repair installation on the same mb should be no problem.
It may not even trigger the activation procedure unless you change more than two of the components MS uses to identify your install as valid, but even if you do need to activate again you should have no problem doing so.
Edited by Enthusiast, 11 September 2006 - 10:52 AM.