In that situation you can do two things, first is to delete the Enum Key in the registry before you close down to swap the drive into the new motherboard, this will make XP look for the new hardware. This is probably not the best way to do it as messing with the registry can screw things up even worse.
 In your situation if you boot from your XP CD then at the first prompt choose to install(not repair at this stage) then at the next prompt it should find your XP installation and ask if you want to repair, you choose that option, it should then work fine after that.

Hood