One of the most frequent causes is a faulty RAM stick. Can you run memtest to check this?
Another possible cause is a damaged hard disk. A test of your system drive is anyway not a total waste of time even if is not the cause of your problem.
Another possible cause is a faulty PSU.
Another possibility is an error somewhere in operative system (it is a real nightmare to find exactly where is the problem :doh: ). Try to reinstall all drivers, often this solve any issue
Do you overclock? Try to reset all BIOS setting to default and see if this solve.