Windows XP has to be re configured to work with the new motherboard. XP has a service called HAL - Hardware Abstraction Layer that is effectively a device driver for the motherboard circuitry. Since you did not install the same model of motherboard, this will need to be reconfigured. That why you are getting the bluescreen.
The second section of the following article in the MS KNowledge Base describes the process. Hopefully your computer came with a copy of XP and not a recovery disk. Also you will most likely need to reactivate XP
If everything works right if should keep all your previous settings, installed software, and data.
How to replace the motherboard on a computer that is running Windows Server 2003, Windows XP, or Windows 2000
http://support.microsoft.com/default.aspx?…kb;en-us;824125
From the article
2. Replace the existing motherboard with the new motherboard.
3. Insert your Windows CD in the CD-ROM drive or the DVD-ROM drive, and start the computer from the CD.
4. Do one of the following:
• For Windows XP:
a. When you are prompted To set up Windows now, press ENTER, press ENTER.
Setup looks for any previous installations of Windows XP on the hard disk and then displays a list of any previous installations that it finds.
b. Use the arrow keys to select the installation that you want to repair, and then press R to select the To repair the selected Windows installation, press R option.
This will start the repair of your previous Windows XP installation.
5. Follow the instructions on the screen, and allow Setup to complete the repair of the previous Windows Server 2003 installation, Windows XP installation, or Windows 2000 installation.
Setup installs the HAL, the IDE controller drivers, and any other drivers that the new motherboard must have.
6. After the repair is completed, reinstall any service packs or hotfixes that you had previously installed.