Ok, first check the health of HDD's with : http://crystalmark.info/software/CrystalDiskInfo/index-e.html
Second thing to check is RAM, you can check it with Windows in built memory diagnostics tool or with : http://www.memtest.org/
The last thing to check is MB, take out all the RAM sticks, start the PC, hear for beep sound, if it beeps then its probably Ok, if it doesn't then its faulty.
And I said you to (Repair-Install the OS with a Bootable Media, like DVD or USB drive), did you tried it ?
Note: Whenever you try to repair install the OS, unplug any other Hard Drive's, after the process is over, plug the drive's back.