Replacing the motherboard is not a big deal if you can get one - contact Toshiba, you may find that it's astonishingly cheap for a recon unit (£25 when I last did it).
But - beware that there is a form of electronic cancer. When one component burns out, the next one in the chain gets it. This happened to me when a floppy drive controller died. Soon, other bits of the board died until it wouldn't boot any more.
First thing to do is isolate the hard drive and make sure your data is safe.
Last thing to do, if you've got Windows XP, when you reboot it will get upset. MicroSoft will give you a new code when you tell them what you've done.
pm if you want more help.