If by rebooting its getting as far as the Windows logo, and then rebooting it sounds like Windows has become corrupt. So not nessecerily a motherboard fault. If you have a Windows CD (or recovery CD) then you should be able to try and recover the existing installation. Simply pop in the CD, let it load up, and select R for recovery. Follow the on-screen prompts and when you get to a C:Windows prompt type chkdsk and let the program do the work.
Good Luck
Urban Kaos, January 2008