Computer initiates shutdown sequence.

I doubt it is a malware. I saw a PC doing it before and the first thing that it points out is that it is a corrupt windows installation which is passing false messages to the motherboard controller. Soometimes simply doing a windows repair or sfc /scannow can fix the problem but you might wanna even try reinstalling windows.
If it still happens after reinstalling windows, it can very well be a hardware fault, for instance the CPU heating up and instructing PC to shutdown.
For more infoo on SFC scannow click here http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Windows/XP/Q_22953945.html

Check to see if all the fans are running - could be overheating. Clean the inside of the case - check for blown capacitors. Check ram for faulty stick (www.memtest.org).

Check the Event Viewer for errors occurring at the time of shutdown.

In the event viewer: At 8:28:49 PM, my SQL server was terminating because of a system shutdown. The latest event before that occurred at 3:19:54 PM when a VNC session was disconnected. That was when I disconnected VNC from work before leaving to go home. There is no event listed as to why the system shutdown had occurred.

I wonder if I had clicked a "shutdown" button by mistake before I got up from the table... When I am at work, I click a screensaver button whenever I get up from the desk. I normally don't do that at home. However, the shutdown button is next to the screensaver button. It's an application called "screensaverswitch" that I'm talking about, which has four tiny buttons down at the corner of the screen. One button starts the screensaver, and another one brings up a shutdown "dialog' which is a cluster of several buttons ("Shutdown", "Restart Computer", "Log off", "Cancel", "Help", etc.)

Is this a laptop? If so, do you run on A/C or battery. If battery, then there may be a possibility that the problem is with the internal battery. Is it getting a full charge? It could be shutting down when there is insufficient power.

Do you use the computer on a UPS or directly connected to the wall outlet? I have seen poor building wiring cause these symptons.

The computer is plugged into a UPS. The UPS does not hold a charge. If I lose power, the computer shuts down immediately. If I lose power for a split second, the computer shuts off immediately and starts a boot up sequence. Normally, there is a USB cable connected between the computer and the UPS, but it's not currently connected. So there is no communication between the two. The USB port in the UPS appears to be broken, and the cable just keeps falling out. So I just leave it disconnected. It is highly possible that I kicked the UPS or the power plugs when I walked away from the chair. But I would think that that would cause an instant shutoff instead of a shutdown sequence.

Try a scan with MBAM.

Is your UPS software installed? Was the USB cable connected when the sutdown started? Is it possible the UPS software detected an abnormality and started the shutdown sequence? If the two are not connected, then the computer likely would just turn off. Since the UPS is not working, you may want to uninstall the software and see if this makes a difference.

Is it possible to replace the UPS and eliminate this as the problem? The UPS software should also detect any power problems over time.

I'm hoping to buy a better UPS when I get enough money. I believe the USB cable is NOT and was NOT connected, I'll try to uninstall the software. This shutdown occurance is so rare, I don't know when it will happen again, if ever.

Is it possible that your Windows sotfware has updated itself and is restarting/shutting down after installing the update?

I doubt that Windows would have updated itself and then rebooted instantly while I had been sitting there using it. That only happens at night. Also, Windows would reboot and tell me that it had installed an update. In this case it was a shutdown with no warning. Before I got up, everything was normal. When I returned (seconds later), programs, folders, and browsers had closed, and others continued closing one by one. Task tray items were disappearing. When Windows had finished shutting down, the machine turned off. I turned it on and rebooted, and everything was fine again.

There is still that possibility I may have inadvertantly clicked on the blue 'X' button on my "Screen Saver Switch" and then clicked "Shutdown" on the popup menu... I just don't specifically remember doing it. See image...

Hi bsaucer,

Did you try SFC /SCANNOW and Windows Repair.
Sounds more like a corrupt windows installation to me. I had this happen to me once, but in my case the PC would just go to sleep mode. I tried everything but in the end had to do a full reinstall of windows to fix the problem.
Make sure you backup all your daya and then reload a clean copy of Windows on it.

I'll just wait till it happens again and then go from there...

A piece of chip stuck in the keyboard must have caused it to go to sleep mode then :)