The following has worked for me with Win98 and it should work for XP:
First the usual Admonition: Back up!
Go to Control Panel(or whatever they call it in XP)/System/Hardware Profile. Copy the "Original Configuration" profile - give it any name (suggest a first letter after "O"). When you reboot you will notice that a menu pops up asking you to choose between profiles or "None of the Above".
Install your new motherboard. When you first boot up and are given the choice of profiles choose "None of the Above"
Windows will now start a re-installation of the hardware
drivers and whatever -- a redoing of the "Original
Configuration." You will get a bunch of "unknown device"
messages and various install messages.
You should have on hand your Original
Disks and whatever other
disks are required to install monitors, video cards,
modems as they may be asked for or you may get the basic
defaults.
And that's it. You should not have to reinstall and reset everything else with this method (fingers crossed!)
(Delete all but "Original Configuration" in the Hardware
Profiles unless you like seeing that menu when you boot!)
(I have not tested this with XP but I expect it to work.)