First, make a backup of your data, just in case.
Then, once the new mobo is installed, insert the XP CD in the drive, and follow the instructions given below (shamelessly lifted from an answer Spikeychris gave in a thread yesterday) -
"When you see the "Welcome To Setup" screen, you will see the options below
This portion of the Setup program prepares Microsoft Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
Press Enter to start the Windows Setup. do not choose "To repair a Windows XP installation using the Recovery Console, press R", (you do not want to load Recovery Console).
Accept the License Agreement and Windows will search for existing Windows installations. Select the XP installation you want to repair from the list and press R to start the repair. Setup will copy the necessary files to the hard drive and reboot. Do not press any key to boot from CD when the message appears. Setup will continue as if it were doing a clean install, but your applications and settings will remain intact.
He will have to re-install all the hotfixes he has downloaded.
This will allow XP to ditch the unneeded drivers, and install the new drivers required for your new mobo. You will probably need to run the CD supplied with the new mobo to finish off the new installation, if it has any 'fancy' features :)