NT based os are not like win9x based os in this regard. If you swap mb, your computer MAY not boot. If the chipset mb drivers were installed for your old board, then most likely it will not boot. There is a way to swap boards without a clean install, however a clean install of the os is the best way to go.
If you do not want to clean install the os, then here is how to swap boards without a problem.
During this process do not reboot the computer when it asks to reboot; tell it later.
1) Uninstall any software that is dependent on your old board such as onboard sound, hardware monitoring software, etc.
2) Uninstall your AV software
3) Remove any devices from device manager that are related to your old board, such as ide controller floppy controller, etc
4) Now shutdown and swap boards. Restart the computer and enter the bios; set CD as first boot device and save settings. Restart with your xp cd in the drive. After setup loads you will get a menu giving the choice of install or repair. Choose to install not repair; then next menu will show any previous xp installs and offer to repair it for you. Let it do so.
When you are finished you will need to install your chipset mb drivers, reapply sp1, DirectX updates, any hot fixes and or patches. You will now need to reactivate xp since a mb swap most likely will require reactivation.
Again a clean install is the better way to go.