It would help to know more about your problem. What is you configuration? What are you doing then the system shuts down? How long have you had this problem? Any recent changes such as program or hardware?
Heat is your computer's enemy. I would check all for grills where air comes in for dust and dirt blocking the air flow. I usually take mine out once and awhile and use a air compressor to blow out all the dust. Heat-sinks and get clogged and lose effectiveness. Just be careful with you fans. Hold them so they don't spin while you clean the dust out around them.