For the last couple of days my computer has been restarting itself when its been idle for a long time. I made sure to turn off anything in properties that might cause this, including hibernate. It still does it, is there any way to know what causes it? I didn't change anything, it just started happening a few days ago.

You might be getting a BSOD (a WinNT STOP error) and it auto-reboots.  To disable this behavior (to get the BSOD and see it on your screen), go to System Properties, under the Advanced Tab, press the Settings button under "Startup and Recovery", and uncheck the "Automatically Restart" button (this all assumes you have WinXP).

Yes thats exactly what I got, what I don't understand is why its doing this when I have adequate disk space and updated BIOS. Is it supposed to get the error when the system is idle or has it been just a coincidence I've never got it while using my computer?

My first thought would be overheating. Bluescreens at random usually indicate a hardware problem. How's your case ventilation?

I had that problem Falcon. But mine was what Stealth said. Overheating. I ignored it totally and in the end, i melted my chip. :-\

It could be a corrupted driver, in which case formatting would fix it.

It could be a hardware problem, in which case you would have to swap stuff till it stops breaking.

It could be a heat problem, check your fans.

One thing that MIGHT give you more insight, if it's a hardware problem, is to boot off a live Linux cd (Knoppix, for instance) and run "dmesg" once in awhile.  That tends to be pretty good for giving good error messages. 

I checked my fans and they all seem to be working fine, I'm going to try to restore my system to before when this started happening and see if that fixes it, if not I'll look into "dmesg", thanks for all your help :).


Edit: Shortly after I posted this it happened again, this time I saw it start dumping physical memory to disk, and then shutting down. It said to disable BIOS caching or shadowing, how would I do that?

Have you done a simple sanity check of your system's operating temperature? That'd be the first thing I would have done after your system started crapping out on you. If it's idling at 75 degrees Celcius or something, you've got a problem.

I suggest you open the crash dump and do analysis on it.