Oh my. Speaking from my past experience with swapping motherboards, it's one of the worst hassles to go through.
You basically need to re-register all of your system components since the board swap wreaks havoc with the Win registry, which calls for a bit of reinstallation or, ideally, a clean install.
First, have you attempted repairing Windows by running the OS from CD-ROM?
I'd suggest changing the boot sequence of your drives in the MB's BIOS so it boots from your CD drive first, with your XP CD inside. You can select a "repair Windows" option from the prompt, which will re-install Windows over your existing installation, but keep your existing settings. It's still not a pleasant or automatic thing however, and will require several restarts and you holding your PC's hand through the process.
Of course, when dealing with touchy problems like this, look before you leap
! Read up as much as you can before you do it. Check out eldergeek's faqs, they can be pretty helpful:Elder Geek
Hope you get your problem sorted out and good luck!