I have switched out mobo and CPU just recently without a re-install or a repair. Everything should go smoothly, the only thing that is going to be new to XP are the controller drivers for the IDE/SATA channels. I switched things out, first went into BIOS and set that up the way I wanted it, and then tried booting normally. When I booted normally I froze, so I restarted and booted into Safe Mode. Got in there just fine and while in safe mode it installed all the new controllers and stuff for the new mobo. Then another normal boot and everything was good.
So I would reccomend swapping mobo and CPU, setup bios, then boot into safe mode to set things up, then boot normally... in that order
Good luck!