To answer the question about changing a motherbaord and Windows XP product activation.
It comes down to the type of Lic. you own. There are OEM, RETAIL, and VOLUME keys.
If you own an OEM key (like you bought a new computer and it had Windows XP on it already) then it is for that hardware only. If you change 3 major components (CPU counts as 2) then it is considered a different computer, and you need a new key.
If you have a RETAIL key (like you went to a store and bought a boxed copy of WinXP) then you can move it from PC to PC. However, if you do change 3 components, or move it from another PC (has to be uninstalled off of the old computer) you will have to reactivate windows again.
If you have a VOLUME lic. you can install it on as many machines as you want, but the total number of computers being accessed at the same time, can not be more than your total number of licenses. Many companies have software in place to keep track of all this. Or they just buy a VOLUME key that covers all thier machines.
Anyway, I can tell you that MS will activate OEM keys on different hardware if you call them to activate. However they can deny you at thier own discretion.