There are a number of possible causes for this problem. The first obvious cause is a failing hard disk, to check this download the manufacturer of your hard disk diagnostic utility from their web site. If the hard disk passes this test then the hard disk is not the problem.
The second possible cause is a failing power supply, to check this possibility substitution with a known good power supply is the only reliable test.
The third possible cause is faulty memory, download a copy of memtest86 and run it overnight to eliminate this possibility.
If you have eliminated all of the above then the only other possible cause is a faulty motherboard.