First

Computers need ventilation. Do not keep your computer in an enclosed cabinet. Even though most desks and hutches have an enclosed cabinet for your PC, it is not the best idea unless your cabinet has an open back. Also the cabinet should not be pushed up against the wall. Open air is a must. Your computer requires constant cool air to circulate through it to keep the internal air from rising above a suitable temperature. A good system temperature runs about 15 to 20 degrees below your cpu (processor) temperature. Temperatures can range depending on the type of cpu you may have. A norm for cpu temperature is around 120 degrees Fahrenheit. A norm for system temperature is around 100 degrees Fahrenheit. Most new computers have a built in thermostat to warn you if your system runs above the norm.

Second

Keep your PC up off the floor about six inches. This is may prevent water damage from flood from broken water pipe or minor roof leaks. Keeping your PC up will also minimize the amount of debris your PC will pull in from the air space around it. Six inches is the rule of thumb. Any small air born debris will be sucked into your PC. If your computer is sitting on carpet, carpet fibers, pet hair, dust, lint and other forms of debris eventually it will be pulled into the system. Keeping a dust free PC is keeping a happy PC.

Third
Safety is also an issue. Keep your computer cords out of your path. Tripping over cords can be dangerous for you and for your computer. Keep your computer away from any form of water. This also includes the occasional coffee cup sitting on top of your computer tower.