Different Mainboard

[Ghostrider]

Have you tried the Sysprep utility in the deploy tools, if your OS is functional you can run it then change the M/B, XP should re-detect on restart, you will have to reneter the Product key and user info etc

[D.Celt]

xunil76's technique is a good one, I would add exporting the registry to it so you don't have to reinstall anything. Also the My documents folder can be moved so you don't have to copy anything back.

Using sysprep is not necessary here I think, if you uninstall the critical drivers (system, ide & display), Windows will default back to generic drivers until specific ones can be installed.