It can be a combination of anything really. Better cooling will guarantee a longer lifetime for your computer parts. A good power supply will ensure that your parts are getting enough power to survive. The PSU will usually give out first if it is being pushed to the limit, but its always smart to have one with some wiggle room.
Also, parts are sometimes faulty from the factory. Hell, someone could get a $1000 Core 2 Extreme chip and have it crap out in a matter of a week or two. Same goes for RAM, video cards, motherboards, etc.
As long as you keep your system cool, have parts that are compatible, and don't overclock like mad, you should be fine.