If none of these fixes get your system limping, you can just pull out the HDD and keep it in an enclosure.

If the problem is a mobo issue, then you really are better off skipping the repair shop and just buying a new computer, since that's what you'd be doing if you got it fixed anyway--replacing a mobo that old means you have to replace everything else, too, because it won't be compatible. Might as well just pull out the hardware that will still be useful (on a 10-year-old computer, that's probably only your HDD, but I don't know what you might have put into it over the years) and use it in your next computer.