The lifespan of the average PC is almost impossible to measure. Our
moderator states he has one that has been in use for the past 10
years. In our office here are some that are between 6 and 7 years old
with some about 2 years old.
But the question also asked about when you could expect the components
to start failing. That is a more difficult question to answer. The
two things that are most likely to fail first are the CRT monitor and
the hard drive. The monitor because the phosphor will become etched
or the electron gun will become less able to provide enough excitation
to drive the screen to its desired brightness.
The hard drive will fail because of mechanical motion, either the
actuator that moves the heads across the surface of the disk or the
motor that spins the disk. Those are two of the most common failures.
The other common failure would be one of the electronic components
such as an integrated circuit or driver transistor. These usually
fail because of heat. If the case fan fails, then the operating
temperature climbs and for every 10 degrees Celcius (about 18 degrees
F) the junction temperature rises in a transistor, its life is cut in
half. If an IC is spec'd to run say 40,000 hours at 30 degrees C (86
F), then if the temperature climbs to 40 C (104 F), the life span is
now 20,000 hours.
This also assumes the IC is powered on for the full time and the
operating temperature can be maintained. What is not counted is
thermal shock, the almost instantaneous heating when electricity
begins to flow. The buildup of dust and dirt in a computer case also
adds to thermal stress as the dust and dirt inhibit the flow of
cooling air and act as insulation to keep the heat in the unit.
Opening the case at least yearly and running a vacuum inside is one
method of reducing this problem. You may also want to use compressed
air, or even the exhaust of the vacuum cleaner to blow out any dust or
dirt the suction misses.
So, how long will a computer last before it fails mechanically? It
depends on the environment. Used for only a couple of hours a day and
properly maintained, almost indefinitely. I'd worry more about my
software. But if a computer is turned on and left running behind the
closed door of a cabinent and not checked and cleaned regularly, then
maybe 8 or 9 years if you are fortunate.