First thing you need to do is confirm that your memory modules are not defective.
Download and run the freeware utility
Memtest86+ V4.20 (25/01/2011). Scroll just past half-way down the web page and download the appropriate pre-compiled bootable image or package.
When you run memtest86+ you should not get any errors if your memory modules are working properly.
If your memory modules pass the memtest86+ testing cycle then I would suspect a hard disk drive problem. You will then need to run a hard disk drive diagnostic utility (downloaded from hard disk drive manufacturer's support website) to determine or confirm if there is a problem.
Please specify more details about your system like make and model of motherboard, CPU, memory modules, graphics card, hard disk drive, power supply unit.