The problem is hardware... either the Power Supply is to weak, RAM is bad, lose or dirty, a power or digital cable is bad or lose, the processor is overheating.
Less likely but still possible would be the HD has master boot record corruption... if checking the hardware doesnt solve the problem, try doing a clean installation... formatting the hard drive will rebuild the Master Boot Record... sometimes rebuilding it from the Recovery Console does it, but not always and you need to format the hard drive.
Check the ram is properly seated in the sockets, and if nothing changes, run Memtest86 or Windows Memory Diagnostic http://pcsupport.about.com/od/toolsofthetrade/tp/memorytest.htm
Checking the Power Supply:http://www.techsupportforum.com/forums/f210/test-your-power-supply-with-a-multi-meter-151526.html
24 pin PSU http://www.pcpower.com/images/manuals/T51XE.gif
20 pin PSU https://www.icmag.com/ic/picture.php?albumid=7678&pictureid=191269http://www.motherboards.org/images/articles/guides/20-pin.jpg