See 7B in left column
http://aumha.org/a/stop.htm

If you change the motherboard, then you probably need to perform a "repair"
installation of XP.

But, in order to do this you will need the retail version XP CD used to
install XP on the PC.    (If you do not have a retail verison of XP, see
below.)

Here are some links about a repair installation:

http://support.microsoft.com/default.aspx?scid=kb;en-us;315341

http://www.webtree.ca/windowsxp/repair_xp.htm

http://www.extremetech.com/print_article/0,3998,a=23979,00.asp

http://www.geekstogo.com/forum/Repair-Windows-XP-t138.html

If this PC came with XP pre-installed, then at best you will have an OEM
version XP CD, or more likely no XP CD at all, just some restoration CD.
Many OEM CDs do not support a repair option, only a clean install.  Further,
OEM CDs may be used only on one PC, which is defined loosely as one
motherboard.  Restoration CDs will usually only work on the same hardware as
was in the original PC, except for a new hard drive.  In the case of a
motherboard failure, you should have gotten a replacment board from the PC
maker.  If you choose to upgrade the motherboard by yourself, that path may
not be supported by the PC maker, nor by Microsoft.   (An OEM license for XP
means that all support, if any, must come form the PC maker.  Such licenses
are cheaper than retail licenses, in part because Microsoft does not have to
answer user questions or solve user problems.)  It would be worth contacting
your PC manufacture's support site for assistance.  But, do not be surprised
if they tell you to buy a full retail version of XP.