Ok, so there I am, idly running around the net when suddenly the computer goes down, the monitor goes into standby mode, the mouse/keyboard/gamepad shut down and the cdrom lights up but isn't spinning. To get out of this, I have to power down the system manualy.

After this, it won't start at all unless I reset my surge protector. However.. it's the only thing pluged into the surge protector that won't start before resetting. The monitor, speakers, etc are still working.

This has happened 3 times, all of which I was online.

All the relevant info:
Win2k
Athalon 1ghz thunderbird
512 megs of ram
80gig HD
250 watt power supply
conexant V90 software modem
Gforce 3 ti500
SB Live Xgamer.
belkin surge master 2 surge protector

The CD-ROM light would suggest a problem there.

Try running your computer with the CDROM disconnected and see if problems still occur.

Also, installing/reinstalling the latest chipset drivers might help.

Could a dead/dying cmos battery cause that? Had the same problem at a friend's place - the cmos battery was dead (the system only 3 years old, go figure!), the power supply just about cooked and the cpu fan wasn't running. Replaced them and no more shutdowns.

I'd lean toward the power supply going out. Seems a bit under-powered for the system.

I would suggest the same thing. I never actually encountered a cmos battery problem before so I have no idea on that one. Yea it does sound like a power supply issue to me at least.