Try re-installing windows. Try booting from the windows cd & reinstalling windows. If practical I would also narrow down other faults by stripping all the extra hardware from your case. I.E. take out all the Isa,PCI & if possible the AGP cards. I would also remove all the memory chips except for the highest capacity ram chip. Tell us a bit more about your machine & what windows version your using. whether it was a "clean" install or an existing windows version.

Windows XP does not like having a motherboard swapped out - as you've just discovered! You can just reinstall Windows as sennetor suggested, or try a repair through booting off the Windows XP CD and selecting the relevant option instead of a fresh install.

Make sure the PC is set to check for a boot disc in the CD drive first. The option is located in your motherboard's BIOS setup (ususally accessed by pressing the del key when prompted).