Help! Changing Motherboard without having to reinstall XP

Hi all,
I dont know if this can be done?
I am going to install a new motherboard and processor into my sons computer and he doesn’t want to have to format the drive and reinstall XP because he needs everything on it and there isn’t enough room to do a complete backup on a partition. Is there a way to do this? Would it work if after installing the hardware and booting it up with the xp diskand going into the recovery console and doing the fix mbr command.
Regards,
Mick Morris.