Since you have no restore discs start with a test. Does Safe Mode work? If not the onboard video is likely failed. And changing the motherboard should cost more than an used/refurb better than this one.
Try booting Linux as well.
I have a problem with my Toshiba Portege S100. The machine will boot up (Windows XP) but screen has vertical lines, and eventually screen goes off.
With an external monitor it does the same. So I am assuming that there is an issue with the video control.
Can I assume this is part of the mother board?
What are the problems with changing the mother board only. I do not have the original XP discs.