Like has been stated, drivers, bad sectors on the hard drive, bad ram, viruses.
... why does a car stop running ? ...lots of reasons,, right ? The better you take care of your car, the lesser of a chance it will break down. Never check your tires, change the oil, check the breaks, coolant, trans fluid, slam it into gear, spin the tires,, you are destined for a major break down.
Same with a pc. You go to porn sites, have p2p software like Kazza, don't clean the processor heat sink out when clogged with dust, bad working fans.
Install "FREE" programs from the internet that tout themselves as virus scanners / registry cleaners / browser helpers, not updating the OS. I worked on an infected pc last week that did not even have service pack 2. ...etc, no spyware or virus programs... the list goes on. Like I said I am not a fan of upgrades. Wipe the hard drive clean including the boot sector, then install your OS. No chance of getting viruses, trojans , spyware or flakey drivers.