Just changing your CPU won't be a problem but changing the motherboard
is a different matter. Sometimes you can "get away" with a "repair install"
of XP when you change your motherboard but it's not considered "best
practice". It is very advisable to do a full format and re-install of your
OS (not just XP but any OS really) when you change your motherboard.
Otherwise hardware drivers left over from the old motherboard will be
hanging around just waiting for the chance to cause you problems.