If you have checked (with an appropriate antivirus and adaware) that you have no virus (or malware), perhaps the problem is not at all malware related. Perhaps the problem comes from an hardware issue.
To be sure: are you using certain type of application when the computer restarts (games, high CPU or graphic card demanding application?)
You could also check the temperature of your CPU and the case (using SpeedFan for example) to see if the temperatures of your computer are normal (a CPU overheated just fail to respond and the computer may restart)

Hope this helps