Although cleaning the inside of a computer is generally unnecessary, in areas that have a lot of dust or may be exposed to insects, it is a wise idea to open the case every now and then and give the insides a solid dusting. Computers are sealed against most contaminants, although dust and small particles or bugs can enter through exhaust fans and ventilation holes in the computer case.

Should I Clean?

Before opening the computer case, determine whether breaking the seal on the case will void the warranty; there is usually a label on the case that provides a warranty warning if it applies. In some cases, you can take the computer to a certified technician for cleaning without violating the warranty.

What Should I Clean?

Clean the bottom of the case of any dust or foreign objects like dead insects or paper scraps. Remove any dust bunnies from the top of circuit boards and moving objects like fan blades. Remove spiderwebs from between circuit boards and around wire bundles.

How Should I Clean It?

Never use liquids or solvents of any kind inside a computer. Use only compressed air to blow away dust and particles. If dust has built up around moving objects, like fans, use tweezers or cotton swabs to dislodge it. Do not shake the computer or bang it around, as this may dislodge sensitive internal components. Do not wipe down the inside of a computer with a damp cloth or use chemically treated rags on the equipment.

Safety First

Always turn off the computer's power completely and unplug the machine from its power source. Electronics are very sensitive to static electricity, so ground yourself before touching a computer's internal components. Also, computer motherboards and expansion cards contain sharp objects that can scratch or puncture the skin, so be careful when reaching inside a case.

Source:

ComputerCleaningGuide.com: Clean Computer Inside

ComputerHope.com: Cleaning the Computer and Its Components

Microsoft.com: Clean Your Computer