I work as a PC technician and this is really one of the troubleshooting problems that would not have a straight through answer.
I will try to give you a troubleshooting strategy to follow in order to nail down your PC problem.
The Problem might be caused by one or multiple of these options:
1. virus problem
2. device driver problem
3. hardware problem
if it is a virus problem I use the following software's to clean the PC up:
1. download and install Antivir antivirus with updates
2. download and run combofix
3. download and install malwarebytes with updates
Please use these programs for cleaning in safe mode and be sure to disable "System Restore" option in windows.
I see that you tried the cleaning process from viruses so yes it might be not a virus problem.
if it is a device driver problem you would have first to disable the "Automatic Restart" option in windows. all device drivers problem throw out a Blue Screen and then restart. and this blue screen will contain the type of error that led your system to reboot. most of the times it could be a VGA driver or a sound driver leading to such reboot.
right click "My Computer" go to "Advanced" tab in the "Startup and Recovery" section press the settings button in their you will find a check box next to "Automatic Restart" option please unchecked it. by doing so your system will not restart immediately if it is a device driver problem and it will show a blue screen with the necessary information.
also if your check your system logs in the administrative tasks you might see things like system error and save dump which is basically a saved info regarding the bug that led to system failure
if it is a hardware problem these are the list of hardware that might led to a system restart:
1. Defective RAM, for this you can download the MemTest86+ program it is free and run it from a floppy or cd-rom and it will test your RAM for defectives, please run the test for at least 3 times. although in your case it would show the errors from the first time.
2. loose CPU fan or damaged CPU fan, your CPU fan might be loosing up somehow and it is not cooling your CPU or your CPU might have dry thermal compound on top of it and needs to be change. or your CPU fan might became old and not rotating in its full speed to cool up your processor.
Please note in case of a CPU overheated problem due to either a loose CPU fan or a dry thermal compound your system will shutdown and will not restart!! and when you turn it back on you will hear a a weird abnormal beeps and you will get a message during your PC POST process saying "CPU Overheated'
3. Defective Power Supply. your power supply maybe got damaged and old and not providing proper power to your PC. you can tell by entering the BIOS and check the supplied power voltage for your hardware like RAM, CPU, VGA.. and things like that. but really the best practice in this case is to replace it with another power supply and test your system to discover that it is bad.
4. Finally it might be system short circuit. you can't tell this unless you pull of all your hardware outside your chassis, plug it outside and run the system for test. if it worked fine outside the chassis without restarting then in this case you need to isolate your motherboard from the chassis metal by replacing your Motherboard raisers on the chassis where the screws slip in or to put an anti static map down your motherboard.
good luck, and I would check first for device drivers problem and defective RAM.
