Been hovering on these forums for a while and thought i might aswell step in for this one
Computer restarts without notifying you of error: XP (not sure about 2000) will, by defualt, reboot the PC after a stop error occurs.. crap for problem determination, right? - It's easy to fix this: Right click My Computer > Properties > "Advanced" Tab > Startup and Recovery > Deselect "Automatically Restart".
The next time a stop error occurs, you can write down the error report and google it, or whatever you need to do - it's a start.
Next, you can monitor various temp sensor readings from within windows (assuming you MB supports it..) using this program http://www.coolmon.org
With regards to you CPU temp issues, it should not run above 60degC in general with any architecture. Optimum temp range is 30-50degC.. obv. colder is better! If it's running at 90degC you're have a major hardware problem, and you best look into what's causing it or, as has already been suggested, you might be looking at buying a new CPU soon.
So, you said your CPU has a variable speed fan? I've seen problems with these before. Is it manually or thermometer controlled? Manuall > Increase the fan speed, it's probably too low. Thermally > Where is the sensor positioned? Make sure the sensor is positioned as close to the processor (the source of heat) itself as possible without obstructing the contact between processor and heatsink.
My advice would be to remove the heatsink\fan, and clean any existing thermal paste from both surfaces, and clean until they both shine. Apply new paste then replace and check the fan rotation is not impared in any way. Dust\wires hanging accross the blades (!)\etc.
Still not working? Get yourself a new aftermarket heatsink & fan. After that you're probably looking at permanently broken CPU or motherboard.
Have fun!