I'm noticing some patterns in PC failure. Here they are.
Batteries fail, causing date errors, or worse, booting problems. These fail after 2-3 years, and can be replaced easily for around $10.
Hard drives fail after around 5 years, causing much pain. Laptop drives can fail after just a year, and tend to develop bad sectors due to mishaps with the laptop. For maximum happiness, replace the drives before they fail, and use the originals as archive drives.
Motherboards sometimes fail, but not on any predictable schedule.
Motherboards can fail if the capacitors dry out or start to bulge and explode. This is more common than it should be, but at the same time, all caps tend to fail after years of use.
(I've also seen small ethernet switches fail due to bad capacitors.)
Floppy drives occasionally fail, but, more often, the floppies fail. They last around 3 years, and then some stop working.
Power supplies fail. In PCs, the power supply is often the culprit when a computer doesn't work. Good PSs last for many years, but the stock ones often fail after around 3-5 years. End users can replace these.
The small switched power adapters used with hard drives and laptops fail, a lot. Most of the time, the problem is that the cable is bent and the wires within are broken. More intensive use, like in a server room, tends to lead to the adapters expiring from overwork. The real fix is to buy gear that has a big power supply with a fan.
Fans fail. These things spin until they start to rattle. They're cheap and easy to replace.
Monitors fail, but it's usually the power supply that goes out. If it's not that, then, the it's a goner.
Mice clog up. Use laser mice.
Keyboards get crumbs in them, and they lose keys. Some have intermittent electrical problems that lead to weird typing problems. These can be cleared up by disconnecting and reconnecting the ribbon cable connecting the keys to the controller.