You cannot change a mb on an nt based os like xp or win2k and expect to boot. There is a procedure to follow to do this, however it starts with the old board still in the system. Since this is not the case, the following may or may not work.
1 Enter the bios and set the CD as the first boot device. Save settings and restart with your xp cd in the drive. Note you must have a full version of XP. Not a recovery cd that comes with pre-installed systems.
2 From the first menu, choose install NOT repair.
3 The next menu will show any previous xp install and give you the option to repair it. Let it do so.
4 Once you are finished, you will need to install chipset / mb drivers, run windows update and reinstall ALL critical updates. I reinstall the video driver as well.