Originally Posted by Northwind_Dagas
If you replaced your motherboard in a PC that already had Windows installed--which is what it sounds like--you will likely have to perform a repair install. Windows chooses the proper bus drivers for your hard drive during installation. When you changed your motherboard, you changed the bus and now Windows cannot access the hard drive(s) with the old drivers.
Boot to the Windows XP CD. Select "Install Windows." Setup will attempt to detect previously installed versions of Windows. It should locate your previous install and give you the option to repair it. Doing this should not delete any data or applications, although some highly integrated apps--such as virus protection--may need to be reinstalled afterwards.