Unless your new motherboard uses the same chipset as the old one,you may need to re-install XP.
However it may be possible to recover your installation.
Here's a couple of guides which explain the procedure.

http://support.microsoft.com/default...;en-us;Q314082

http://www.intel.com/support/chipsets/iaa/harddrive.htm

http://www.anandtech.com/guides/viewfaq.html?i=113