Go to your control panel and hit the icon for Administrative tools. This will bring up another folder. Click on the computer management icon and look through the system and software events tab. Strange events will have a yellow icon beside them. Full blown errors will have a red icon.

These should give you some idea what problem your computer is having. If there are no errors listed then it might be that your computer is overheating. Emergency shut offs due to temperature warnings tend to happen without leaving any indication behind. All it takes is enough dust on the CPU heatsink to block airflow through the fins and you can have a major problem.