With a desktop computer, you might see as little as three years or as many as eight, depending on the quality of the machine, how often you upgrade and how well you care for it. The longevity of a computer depends on the parts within -- and in a desktop computer, that's a good thing. Desktops on average last longer than laptops because portability leads to accidents that can cause component damage, and because lack of airflow leads to overheating in the components. Also, when a component breaks on a desktop, you can usually replace the part at a lower cost than replacing a computer.
The Whole Computer
If you're looking at the computer as a whole, with no interest in upgrading or replacing parts, expect to get at least three years out of the average desktop computer. Jill Duffy of "PC Magazine" advises that even a low-end desktop computer should give you about five years of solid use before it begins to break down. If you're using a higher-end computer with better components, that number could go as high as seven years.
The better you care for your desktop, the longer it's going to last. Don't leave the tower on the floor, and avoid placing it in corners and near obstructions like furniture or curtains. All of these can lead to overheating, both by introducing more dust and debris into the computer, and by blocking the vents that help control airflow. Clean out your computer regularly with a can of compressed air to remove dust. Keep your computer at room temperature and avoid areas with high humidity whenever possible; extreme climate conditions can cause your computer to wear out faster. Keep an up-to-date antivirus program on the computer and regularly remove useless files to ensure that your hard drive continues to run as efficiently as possible. Install operating system updates when they're released to improve performance and security.
Of all the parts in your computer that are likely to fail, the hard drive is the most common. The type of hard drive typically installed in a desktop computer is a hard disk drive, or HDD. These drives contain spinning platters that store all of a computer's data, which is modified with moving heads that read and write the data. All these precisely designed moving parts can be damaged by sudden movements, extreme heat and time. An HDD is inexpensive, with prices well under 1 dollar per gigabyte; some run as low as 5 cents per gigabyte.
A solid state drive, or SSD, uses flash memory similar to that of a USB flash drive, and uses no mechanical parts. These drives are smaller and more expensive -- typically more than 2 dollars per gigabyte -- but will typically keep up peak performance for five years or more, before beginning to slow down. These drives are faster and more responsive than HDD.
Of all the components in your computer, your RAM is the least likely to break down. According to Lexar, which manufactures RAM and other memory products, your RAM could last decades -- much longer than the computer it's installed in. However, RAM can become corrupt by regular computer use. RAM failure can knock out your entire system, but the sticks can be replaced as easily as a hard drive.
The lifespan of a motherboard has less to do with failure and more to do with obsolescence. The motherboard facilitates communication between all the components that make up your computer, and it has certain limitations. While upgrading your RAM can breathe a bit more life into an old computer, your motherboard is limited in how much RAM it can process and what type of RAM it uses. Motherboards are also limited to a specific type of processor, locking you into the family of processors for upgrades. As your computer ages, you may find that your motherboard can't handle the newest and fastest components. Eventually, this begins to limit the software you can install.