How long does a desktop PC usually last?

The vast majority of PCs get replaced because of these reasons:

• They are too slow for the current operating system or for the current applications
• They lack sufficient RAM
• Their disks are too small

Failures are rare but they can occur at any time. There is nothing you can do other than preventing a build-up of dust (which could cause overheating).

While there is little you can do to extend your machine's life expectancy, you can do a lot to protect your data - see here for a drastic example.

First, note that slowly malfunctioning is rare. A computer usually completely fails, abruptly.

Second, note that although we may think of a computer as a single thing, it's actually a collection of components, and any one of them may fail. They never fail all at once. And most failures, even if it's a small inexpensive component like the motherboard battery (see below), mean the whole computer won't work.

So if a component fails, you seldom have to throw away the whole computer. You can simply replace the failed component. It's like a car; you can keep replacing components forever. For example, if the car's battery fails you don't have to buy a new car; just buy a new battery.

So to answer the "how long does it last" question, the answer is it depends. It depends on what component fails and how easy and how expensive it is to replace it. If the motherboard battery fails, you can buy a new one for under $5US, and it's easy to replace. If the CPU fails, depending on what model it is, it may be expensive or so old that it's hard to find and you therefore need a new motherboard as well as a new CPU.

Again, it's like a car. It can last forever if you keep replacing parts as they die, but at some point you may decide that it's cheaper and less trouble to buy a whole new one rather than replace the last expensive component that dies.

And one remaining point: at some point, whether or not any components have died, you may find that you want a new operating system, new applications, or new peripheral devices like printers that won't work with your old hardware, or if they do work, they work so slowly or poorly that you are unhappy with the  results. If I've counted correctly, I am now using my eighth desktop PC, and I never replaced any of the previous ones because they died; I replaced them all because I needed or wanted a new computer.

A desktop PC lasts a long time, if properly maintained.  I look after 150 win7 client computers and have been doing this for 15 years now.  Many of my clients' desktop computers are 6, 7, 8 or even 10 years old.  (Notebook or laptop computers last about half as long.)

The components that are most likely to fail:

• DVD drive.  Easily replaced for about $20 and a few screws and plugs
• Fans.  They get noisy as they wear.  You can make them last longer if you do not leave your PC running 24x7.  Fan replacement is actually quite cheap (they are about $10) but requires some handy-man ingenuity.
• Video card fans.  Only way to fix this is with a replacement video card.  Although some of these PCs have video outputs on the mainboard which work just as well as the video card once you remove it.  If you need to replace the video board, insist on one that has no fan on it.
• Hard drive. After age 5 test it at least annually using the hard drive manufacturer's drive tester tool.  Replacement from a hardware perspective is incredibly easy. Cost is about $50. But replacement means complete re-install of Windows 7.  That actually has an amazing affect.  Particularly if your replace with a 7200rpm drive.  Performance improvement will be stunning.
• Over-heating due to failure of the heat-conducting paste between the processor and the heat synch on top of it.  Easily corrected for about $10 for a tube of the paste.
• Failure of the BIOS battery. It is a standard 2032 coin cell you can buy at the dollar store for $1.  Lasts 5 years or more.
• Power supplies fail, but not often.  Cost is about $60, but you really have to know how to do this.

The only valid reason to replace a desktop PC is either a failure of the mainboard or the resources in the system are just too slow to cope with what you want it to do. 

Memory can be a limitation due to the inability of the mainboard to accommodate sufficient memory.  Although, some of my clients use Win7 32 bit and have 2G of memory and their machines run just fine.

Make a habit of regularly (annually or more) opening up the box and blowing out the dust with a can of compressed air ($3).