When you install Windows XP, it registers a specific "hash" number (the long xxxx-xxxx-xxxx-xxxx-xxxx-xxxx) that is based on your major hardware, such as your motherboard. When you simply move a hard drive to a new computer, it cannot find the hardware it was registered with and will not boot.
DO NOT take my advice until someone else will verify, because I don't want you to have to reinstall the OS completely.
What I would do is boot to the XP CD (assuming you have one), and run a repair install. It will rewrite the OS, but leave all your data intact. Once the repair is done, Windows should boot fine. You will have to re-activate Windows XP and chances are you will need to speak to a nice Indian fellow from Microsoft. Just tell the rep that you replaced the motherboard and they will give you a code to activate.
I have been in the field for almost 10 years, so I am very well versed PC hardware and software. I just don't want you to get to a point where you are forced with a clean install. Hope this helps
