Verify the product code you used to be sure you entered the correct letters and numbers. If so, then there two possibilities:
1) Your product key is an OEM key that came on a pre-assembled computer, like Dell or HP. If that is the case, try the support site, or call their technical support for resolution. Changing the motherboard will likely cancel your XP install, since Dell, HP, etc. use OEM XP, which cannot be transferred to another system.
2) MS has identified your product key code as stolen, compromised, or possibly, pirated. You would have to buy a new product key code; it is included in the box with a new XP CD.