Does this happen randomly or only when it's been running a while ?. Have you overclocked any components or fiddled with your bios settings ?. Are your PC innards clean or full of dust ? .
Use another PC to make yourself a Memtest 86+ boot CD....(http://www.memtest.org/)
Boot from this (NOT your hard drive) and allow at least 3 runs. This will give your PC's 'guts' a good test without running any OS.....and work onwards from that base.