Download a program called MemTest86+ and burn it on a CD and boot with it right before you go to bed. Make sure it is working and let it run for 8 hours while you are sleeping.
Also, update your drivers if you have any that are out of date.
Is there anything special you are doing most of the time when this happens? Gaming, for instance? If so, then start with your video card drivers. If your drivers are the newest then you can try installing older ones instead.
- Edit - Also, boot up in safe mode and see if it is possible for you to crash it there. This helps very much for you to narrow it down between a hardware problem and a software problem.