I joined this form because of this problem. I have a Shuttle FNG2 with almost the same arrangement as yours - AMD 2100, dual 256 DDR RAM, onboard Nvidia, with Windows 2000 OS. Almost since the beginning it has had spontaneous restarts, maybe up to 4 times a day. It also has had trouble with a couple of the Microsoft Office programs. I've changed out memory, observed the CPU temps, reloaded the OS and surfed the other forms without much luck. I'm starting to suspect it is the Nforce 2 chip or the AMD 2100 to be the culprit. I did upgrade the BIOS with no effect. The next thing will be to try replacing the CPU with a different AMD unit, probably a 2600.
What I have read on other forums is that it could possibly be related to the OS itself. I've run Sandra on every possible component with no problems. Still keep kicking off every so often.