I have found hard drives to be extremely reliable (wait! what was that noise?), and have attended to very few failures.
Motherboards come top of my list, although often the cause is user misuse and/or abuse.
CD ROMs next, followed by fans - graphics card fans seem particularly glitchy.
All in all I think that computer hardware components are among the most reliable devices on the planet - there are few machines that can run for such long periods (often in less than ideal environments) with so little attention.