SInce it coud happen due to multiple reasons, I would suggest you to try the below points:
1. Check the properties of the System, CONTROL PANEL --> SYSTEM --> ADVANCED SYSTEM SETTINGS --> STARTUP AND RECOVERY, make sure 'Automatically Restart' is NOT checked and you write to the system log. That should stop the auto-restart.
2. If it still occurs, the look in your BIOS. Under Power there might be an auto restart as well. If there is, turn it off.
3. Still happens, look for a Power Failure of the PSU or overheating. Clean the inside of the case and fans.
4. Lastly, look in the EVENT VIEWER (RUN --> enter EVENTVWR.MSC) for possible errors at the time of the reboot. If you find anything, post it in the comments and I will get back to you.
I kept getting the blue screen with Driver Power State Failure error message and then my computer would restart. I found your product and it did not happen any more. I just purchased an anti-virus (kaspersky) and now it is doing it again. It is so frustrating. My computer is only 6 months old. It is a Asus desktop.