Random reboots and BSODs are caused by a few things:
- RAM needs checking
http://www.memtest86.com has a good app for running RAM checks
- loose cards or cables
- drivers are really off
-- reinstall drivers properly (remove, reboot, reinstall)
-- do not use omega or beta drivers if possible
- heat issues
-- get some canned air and clean out the cooling fins and fans
-- check to make sure that all fans rotate freely
- viruses or malware
-- make sure your system is free of nasties and that you have all the latest Windows updates
- and occaisionally the power supply is flaky and starts to break down under load
-- this one is subtle and hard to diagnose