Recently, I got my parent's old broken HP Pavilion 6355 that they had bought in 1998. Basically they said that if I could fix it I could keep it. I opened the thing up to see if there were any broken wires, connections, etc. inside the tower, and lo-and-behold there were huge clods of dust. I'm talking shag carpeting here, people. I did a little research online and found out that dust is the #1 cause of computers breaking (Not to mention T.V.s, video game consoles, and lots of other stuff).
So, basically, in this 'Ible you will learn how to properly clean out a computer tower.

Step 1: The Tools

For tools you'll need:
-1 Phillips head screwdriver (Standered 2mm should work fine, but it all depends of your computer)
-a dish for the screws
-a long metal object (Scizors, metal pole, etc)
-a can of compressed air, or a weak air compressor (Such as one thats going bad)
-discharge band (Optional)

Step 2: Opening the Tower

Ok, before your open up your tower, look online or in your computer's manual and try to figure out which side is the correct side to open. Some computers open up on both sides, but one side has the motherboard in it and you could damage it.
After you've figured that out, find the appropriate screwdriver for your casing.
Turn off your computer and unplug EVERYTHING attatched to it. Monitor, AC power adaptor, keyboard, everything. Let it sit for about 5-10 minutes so that it can discharge any left over electricity.
After that time period has passed, it's time for you to discharge. Put your metal object on the ground and gently brush your hands along it a couple of times (Or attach your discharge bracelet around your wrist then to the ground). This discharges any static electricity that could harm the inner workings of your computer.
Now is time to crack that sucka open. Unscrew it and follow the manufactorers instructions to remove the side of the casing. Good, now your computer tower is open and you can begin the operation!!

Step 3: Begin the Operation!

Now, for this step, do NOT take anything out of your computer unless you know what your doing. All it will take to clean out your computer is your can of compressed air (or weak air compressor). Start blowing. You want to try and get as much of the gunk out as possible, so don't be afraid to leave the air flow on one area for a minute or so.
Once your satisfied that you've gotten as much dust out of the cabinet as possible, look in the back for your fan. Tilt your computer tower so that the backside (fan side) is facing slightly downward. Give it a couple of shots of air.
Finee!! Reasemble your tower, screw in the screws again, plug everything back in, and test. Your computer should be running a bit faster, be less noisy, and overall perform better then before.