Helping people with computers... one answer at a time.
Computers can be configured to shut down after periods of inactivity. But if it happens without having configured it, reasons aren't as clear.
When my desktop PC is idle for a time, it shuts off. At first, I thought it was a thermal issue, but the unit doesn't shut off when it's being used, even for extended periods. That led me to think it was a power-management glitch. I figured that disabling system standby and/or hibernation would work around the problem, but when I checked the settings, they were already turned off.
I don't have anything very specific, but there are a few things I'll throw out as places to look and perhaps some items that hadn't been thought of yet.
The first thing that comes to mind is your screen saver. Particularly if "after a time" is a relatively consistent amount of time. Normally a screen saver wouldn't turn the computer off, but a screen saver is software after all, and could have a bug that's manifesting on your machine.
This certainly sounds and feels like it could be a standby or hibernation issue - though if either you'd be able to tell right away when you turn the computer back on. If it resumes immediately it went into standby, and if it says "resuming from hibernation" that's pretty clear. Even if hibernation and standby are turned off, for a long time I ran into a problem where they would automagically turn themselves back on.
Another place to look is the ACPI (Advanced Configuration and Power Interface) power settings in the BIOS. Power management is, at its root, a BIOS initiated and managed thing. Perhaps turning that functionality off in the BIOS, or at least twiddling with the settings there might affect the issue.
There are services that kick in after inactivity. The indexing service is one (you can turn it off), and in Vista I believe there's a bunch of defragging stuff that happens in the background after some idle time. In either of these or similar cases it's possible that the service is causing a problem that would result in the machine shutting down.
In fact, because of the possibility of services kicking in at idle time, it actually still could be a thermal issue. The services might be driving the machine harder than you do when you use it normally, and thus the machine might be experiencing higher temperatures. If it's something you can watch, try running process explorer and see if some application is using up all CPU prior to the machine shutting down.
It certainly wouldn't hurt to run a RAM diagnostic anyway.
You might have a look in the event log for anything suspicious. Error messages or messages of other activity that happen around the time of the shutdown might throw some interesting light on the situation.
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