computer shut down problem

It may cause due to your ram ... please check it and change its slot

and do complete hard drive scan

Faulty Fans, Dust and Grime build up on the Heatsink restricting air flow. I would give the inside of the case a blowout with compressed air. When blowing air through the fans make sure that you physically stop them from spinning, as they may generate power and blow something up. Remembering to ground yourself by placing the back of your hand on the Power Supply Unit and not moving your feet. By not taking this precaution it is possible that you could inadvertantly cause damage to the PC from an electrostatic discharge. Then remove the Heatsink, giving it a thorough clean and reseat the CPU applying new CPU grease. If the Fans spin freely when you give them a spin they are probably OK. If there is resistance replace them.

Clean the golden edge of each memory stick with a soft rubber, remembering not to touch the golden edge of the memory stick. Check with one stick at a time, remembering to disconnect the power from the PC.
Also check Capacitors around the CPU for swelling or bulging.

Check the memory.
You can test the memory by running Windows Memory Diagnostic that can be downloaded from http://oca.microsoft.com/en/windiag.asp. If memory problems are found, try re-seating the RAM. If it doesn't work, replace the defective RAM.

Test the Power Supply Unit.

Download Prime95.

http://www.mersenne.org/freesoft.htm

Prime95 is used to put your system at full load. You want your system at full load when checking your voltages to ensure your PSU is up to spec. The whole point is to make sure your PSU can handle what your system asks of it. When it is installed run it and go to Options and run the Torture Test. Run the Inplace FFTs (Max Power, Heat and some Ram).
If your PC restarts during this test you have a faulty Power Supply (PSU) and it will need replacing.

Download SpeedFan and check the Voltages and Temps while Prime95 is running.

http://www.almico.com/sfdownload.php

You will want your 12-volt rail to be within 11.52 to 12.48 during load. This means when you are running your cpu at 100% you do not want to drop below 11.52 or you may experience stability problems including but not limited to system restarts and Windows crashes. For the 5-volt rail, you want it to be within 4.8 to 5.2 to be within the 4% range. As for the 3.3 volt rail, you want it to be within 3.17 to 3.43

Video Memory Stress Test 1.4
http://www.majorgeeks.com/Video_Memory_Stress_Test__d5896.html

Note: A damaged or insufficiently charged internal battery can corrupt CMOS or BIOS settings. It can also cause all sorts of wierd things to happen.