and let us know how you get on.
Check the memory.
Sometimes random lockups can be attributed to the computer memory starting to fail. 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.
Video Memory Stress Test 1.4
http://www.majorgeeks.com/Video_Memory_Stress_Test__d5896.html
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