ghimpe is correct, but I checked link he provided and found it lacking. When I change motherboards to a different model, I do a repair install of Windows. It's a good idea for Win9x and Me, but mandatory for Win2K and XP.
Here's a link with detailed instructions for a repair install of XP. You will have to reinstall any Service Packs or updates that are not on XP CD.
If you have installed XP SP2 and it is not on XP CD, you will need to borrow an OEM CD with SP2 or slipstream SP2 onto your XP CD.
Repair install:
http://www.michaelstevenstech.com/XPrepairinstall.htm
Slipstream SP2:
http://www.helpwithwindows.com/WindowsXP/winxp-sp2-bootcd.html
Dalton