Be prepared for a disappointment.
Like the Irish story of someone asking the way and being told - "I wouldn't start from here if I were you Sir."
The new motherboard will require its own drivers, and XP on the old HD, will find itself mated (in effect) to a 'new computer' and demand reactivation if not reinstallation.
I would hate to interfere without my own copy of XP, and you may now need to buy one.
Some manufacturers put the necessary recovery files in a seperate "Drive" not always visible in a quick search.
You will need a lot of help here, but I hope this will drag your thread up the list again