Possible Shut Down Virus

[computer_user]

Hello,

I've got a problem with my PC. It's a Compaq Presario SR5710F and for it is experiencing several problems - namely shutting down when it's been idle for longer than 15 minutes, and shutting down at start up. For the former, to prevent a shut down when I'm not actively using the computer, I activate a long running program; thus far this has worked. For the latter, everyday for weeks now, the computer will shut down at start up. Then a couple of things can happen:

1. Upon pressing the power button (on the second attempt), Windows will start normally

2. It will shut down again and I wait a few minutes before going again which could then lead to Windows starting normally

3. It will open with an option for repair or starting Windows normally. I always choose "repair" when presented with those options. Unfortunately, there are times when it shuts down even in repair. When this happens, I wait a moment then press the power button again. This usually causes the computer to again give the option for starting Windows normally or starting repair. With repair, the computer runs a check and then starts normally. Yet, the next time I must turn the computer on, it happens again.

Hardware diagnostics, which runs every Sunday, never finds any problems with my computer. I've performed system restore (setting the computer back to the earliest possible date) but to no avail.

Could a virus be to blame? I've got Avast! installed on my computer and it's been very good at detecting and removing malware and viruses that have tried to install themselves on my computer, but I wonder if something could be getting past the tool this time.

Can you provide assistance?

Thanks.