Have you run any virus scans? Or Malware scans? It could be your computer has a bug that's telling the computer to shut down.

I'm with Drac and Jenny, something is giving the computer the shut down command.

There's a setting in the BIOS that tells the computer how hot it can get before shutting down.

Go through the application logs in Event Manager, you should see what’s triggering it there.

Some UPS (Uninterruptible Power Supply) comes with software that will automatically shut down the computer if the power goes out. If something is going wrong with the UPS, it might do something like turn off the computer when there is no power outage. Do you have a UPS and did you install software for it?

Another possibility -- the power button itself might have a bad connection -- acting as if it's been pressed when it hasn't.

If you want to check your Event Viewer in XP, here are instructions
https://support.gateway.com/s/SOFTWARE/MICROSOF/7509594/750959425.shtml

Does it "shut down" or just power off?

Where do I find the Event Manager?

Memory okay. 0 errors. Norton Antivirus 2012 complete system scan only came up with a few cookies.

Will check out Bios for overheating next and Event manager log when I can find it.

Edit: Sorry Jenny. Did not see your posting when I replied. Will check it out.

Usually I don't know when the system turns off. It either does it when I'm out and have left the system running or I've fallen asleep and wake to find the computer off. The last time occurred while I was watching TV and the shutdown music happened and then it was off.

It doesn't occur at regular intervals or times which makes me think it may have something to do with overheating. But then I've played games for hours without anything happening. This only happens when the system is sitting idle.

Will check Bios and your site Jenny and let you know my findings. (I don't have a UPS.)

Because the computer is shutting down as though its been told to shut down, is it possible there's a setting that's been set to shut down the computer after so much idle time?

I ask because some have said if this or that is going bad the computer just shuts off, not shuts down.

If there is a setting that has been changed I can't find it. I'm going to check out Norton Antivirus to see if any settings were changed when it updated. I know there is a setting there that will cause the system to turn off after a complete scan, but I'm pretty sure I told it to stay on.

I glanced at the Event Log and didn't really understand it. I think it said that Adobe tried to do something and something didn't complete and there might be file corruption.

I ran chkdsk from within Windows but it didn't tell me anything. There was no report about corrupted files or any problems. I did have it set to try to recover any damaged sectors or whatever. I didn't tell it to automatically repair as I wanted to see what the damage was first. Either there wasn't any or Windows is trying to keep it a secret.

I'm beginning to think I'm going to have to do a reformat to recover from these behavior problems with my PC. I don't want to do it because it can be a nightmare trying to get it updated to proper working order.

I think it's probable that I caused these problems by making changes in the system. Deleting files that may have been necessary, etc.

If I do decide to reformat at least I have my external hdd to backup any data I don't want to lose.

As long as you've got good backups, that's the best route. When I backed off Windows 8, that's the route I took, the PC hasn't worked this well in years.