Most Common Reasons that Windows Blue Screen:
Malfunctioning Devices Connected to it (Flash Drive, WebCam, etc...)
People who still use IE 5 and don't realize there are better browsers which are safer.
Downloading "Hax" and random research from shady sites which contain Trojans
Opening every God Damn Spam email/Falling for Scams
At least my school isn't completely handicapped, and allows Firefox as a web browser.