May be able to help knock out a few options: I am running freelancer on a win98se system,(with DX9 loaded), Gainward MX440 AGP8X card, Duron 1300 cpu, 512 mb ddr2700 all sat on a GA-7AXP. Stable as a rock.
So, its unlikely to be your Graphics card, DX9 or OS.
Is the spontaneous reboot only in XP? or do you see it in all OSs?
Have you checked system temps? (I find that I only push about an additional 5C into the CPU even after a long session).