Computer shuts down by itself

Hi,
this problem just recently appeared: My PC sometimes shuts itself down without any warning. This doesn't happen very often at least yet. Has happened three times so far, all within this year.

I've tracked down that the first 2 reboots happened while the machine was running a daily virus scan. I was not using the machine when these shutdowns happened. Just afterwards found out that the machine was off even though I left it on. The third shutdown, on the other hand, happened while I was using the machine normally. No ongoing virus scan this time.

The shutdown happens just like when I press the shutdown button in windows XP. The applications close, "windows is shutting down" screen appears etc. No error messages of any type. Event log doesn't seem to show anything special, just a normal shutdown.

As it sounds like something a virus could do, I ran a full virus scan in safe mode, but found nothing serious.

Does this sound more like a software problem, of a problem with the hardware? Note that I've had a video card fail on this same machine and before the failure the video card's drivers gave error messages about the card not getting enough power. The new card seemed to work fine so I thought it was just the failing card doing some false alarms. The power supply should be good enough for the machine, a nexus 400W PSU.

 

one of the solutions my be is this one->
System Properties->tab Advanced
Startup and Recovery tab ->Settings
System failure check out Automatically restart
that should solve the problem for a while =)