Step 1: Dust Off and Clean Your Computer
Dust is a major killer of speed if you haven't cleaned it up in over a year. This can block filters, and prevent air from flowing through the computer. It causes the machine to overheat. It also affects disk drives and can even block USB ports and interfaces. Dust also acts as an insulator which can add to the heat buildup. More importantly, dust can also cause a short in circuit boards and integrated circuits causing computers to crash and, in some cases, even catch fire!
Be careful when removing the dust, use caned air to blow it out & a vacuum to get the lose clumps around the computer that is NOT INSIDE IT. NEVER use a vacuum to clean out a computer, as it will cause electrostatic shock. Companies sell special vacuums designed for computers. Only use compressed air for the computer, vacuum for the area the computer is in.
Also make sure that you touch the outside case before you start working to discharge your body static. You can damage the computer boards if you cause a spark! The main thing that needs to be cleaned regularly is the CPU fan. Dust on the CPU fan is the number one reason for overheating and slow performance. Also, lots of dust between pins and connectors can short the circuit, so it’s always a good idea to clean the PC regularly. It’s most important to do this before installing any new hardware.
After you are done cleaning up the inside of your computer, be sure to also clean up your mouse and keyboard. I would definitely dust off your computer at least once every 6 months.
For those who are more wary of sparking the system: You should never open your case without being correctly “earthed” with a conductive strap and bonding point. You can look into purchasing them, and can see a picture below. They aren't very expensive.
A Quote from a commenter: "The reason you should not use a vacuum cleaner is that this can (and has been shown in many cases) to cause a static discharge through the components. It is fine to use a small vacuum on the keyboard (not on a laptop) but not on the case. The problem faced is that many people believe that they have not caused damage, because their PC often keeps working. The problem with static discharge is that it only occasionally causes critical failure of parts. The OS often error handles problems caused by damaged components, as (to an extent) the components themselves do. The side effect of this is a slower, less efficient system. To place static electricity damage in perspective - when sitting still at a desk, your body generates approximately 100 times more static electricity than is required to damage computer hardware. A vacuum cleaner and the flow of air creates a huge amount more."