To keep your computer in prime condition, be sure to open the chassis on a regular basis, and clear out any dust buildup inside. *Be sure to unplug the power cable first!*
The best way to do this is with compressed air. You can buy cans of this at almost all computer or office supply stores. Electronics attract dust like a magnet, so the buildup can get pretty bad if you don't maintain it. If you have pets or live in a dusty area, it can build up even faster. If dust clogs up the internal fans, it can cause overheating and thus lower the life span of your processor, motherboard, power supply, and other components. You may also want to remove the RAM modules and blow out any dust in the receptacles, and make sure there is no dust in your USB and other ports. I've seen many computers with dead insects in them, which is never a good sign. If this happens to you, set some bug bait traps near your tower, and keep all food and drink away from your desk. Always check to make sure all internal connections are tight before putting the cover back on your PC.
To help keep moisture away, make sure your tower is not near a window. Even if the window is closed most of the time, condensation occurs on the glass. If you do have the window open, you risk rain or other water coming in. If you plan to put your computer into storage for any amount of time you should wrap your tower in plastic, and make sure to put it above ground level, in case of flood.
Last, but not least, if there are thunder or lightning storms, do not just turn off your computer. Make sure it is physically unplugged from the wall. A nearby lightning strike can fry your system even if it is turned off!
These tips should help you get the maximum life span from your desktop PC hardware. There are also other things you can do to keep your hard drive data healthy that I will not cover in this article, but you can find many other great tips at here at Bright Hub.