Auto shutdown and restart

What has changed on this system?

If this is a new install of Win2000 and it's done this since installing, then look into bios upgrades for this systemboard and make sure Win2000 is supported with the BIOS version.

If this system has run Win2000 successfully before, and this is a new problem... then look into:
1) potential overheating and or power supply issues.
2) possible beginning hdd failures... so boot to recovery console and at the c:> prompt, and type:
chkdsk /r
That will find and repair any problems from the previous boots and should fix the software side.

The failure of the system may have a lot of causes. The very usual cause is because of the RAM memory - usually you receive a blue screen with "Memory Dump" message. Other causes may be: overheating of the processor (cooler stop functioning), HDD bad sectors (the swap process fails and you also get a blue screen) or video card overheating.
If I would be you, I'd first open the case and see if coolers are functionig. Then I'd change the RAM memory with new ones and see if I get the same behaviour. If yes -> I'd check the HDDisks.
By the way, in MyComputer > Properties > Advanced > Stratup and Recovery -> disable "Automatically reboot" and chech %system%\minidump to see what is happening.