Since this is in the XP forum I'm guessing SP Pro is a typo?
If you didn't reinstall the operating system after installing the new motherboard this would explain why you are having trouble.
When you take a hdd with a Windows operating system installed from the computer it was originally installed on and install it in another computer you are asking for major problems. The excerpt below is from a Intel
article which describes in detail what happens. The article also mentions a reference to an article by Microsoft, it can be seen here
Moving a hard drive with Windows already installed to a new motherboard without reinstalling the operating system is not recommended.
If a hard drive is moved to a new computer, the registry entries and drivers for the mass storage controller hardware on the new motherboard are not installed in Windows for the new computer you may not be able to start Windows. This is true even if you move the hard drive to a motherboard with the same chipset, as different hardware revisions can cause this issue as well.
Additionally, moving a hard drive to a new motherboard may not exhibit any errors until you install new IDE drivers. This is because each chipset uses a different Plug-n-Play (PNP) ID to identify it. If you change your motherboard, your registry will have multiple PNP IDs (for the old hardware as well as the new hardware). If there are multiple entries in the registry, Windows cannot determine which hardware to initialize and therefore fails with a STOP error.
If this is the case, you will need to do a fresh installation of XP.
Did you use the CD which came with the motherboard to install the drivers?