Your Computer keeps restarting without warning which is very annoying! What could be wrong? Most often it's caused by either corrupt software or heat. One of the most common reasons for a computer to shut down suddenly and re-start is recently installed software. This could be device drivers or updates to programs and utilities. Even updates to your Firewall, for example. Or perhaps your Flash Player. To check for recently installed software, open Control Panel and click on Add or Remove.
Tick the box at the top 'Show Updates' and in the 'Sort By' box to the right of this, choose-Date Last Used. Run through the list and look for 'Installed On'. Apart from Microsoft Updates and Hot Fixes, do not remove these, look for any others on or near the date your Computer first started to restart by itself. Make a note of the Program this update was for, and then uninstall it. If this fixes your problem you can then attempt to re-install the update. Always go to the Software makers own site to download updates, rather than a third party site.
So for Flash Player, for instance, get your update direct from their site. The second most common cause for a Computer to suddenly shut down and re-start, is overheating. Does your PC have plenty of ventilation? Are all the internal fans working? Do you need to add an extra Case fan? Also, blow off any dust from the fan blades and case vents. Fit a bigger CPU fan, if you can. You could also try cleaning the CPU and Heat sink and apply a fresh smear of Thermal Grease. Viruses and Malicious Spyware can also sometimes cause similar symptoms. Make sure your Anti-Virus and Anti-Spyware programs are up to date and run a Full System Scan, with both.