Blue screens are usually caused by either memory problems or driver problems particularly video drivers. The first thing to do is to run a memory test program such as memtest86 overnight. If it fails the test you have found the problem. Faulty memory may not be the memories fault but could be caused by a bad power supply, incorrect voltage to the memory, or overclocked memory. If the memory test shows no problems the suspicion then falls on the video driver, try rolling back the driver or upgrading the driver to the latest version.