Install it and run a full scan
If it finds nothing and NOD finds nothing then you have a hardware problem.
It could be your CPU overheating, Power supply going out, or RAM casuign errors.
I would open you side panel and check to make sure that the fan is running over the CPU and make sure the heat sink is dust free.
If so the you coudl try a new layer of thermal past between the CPU and the heatsink.
If it is still restarting you could buy a reasonable power supply and hopfully that will cure it.
Is it doing it when it is at idle or when you are putting the processor under a load?