I recently did a job for some one and within part of this job was having to clean out the inside of the system, I normally do this anyway and so should you every now and then, if you read all this you will see why as I ran a test to show people just how much dust can be a breading ground inside your computer case and this slows down your pc in a very big way because of the massive increase in temperature.
First of all I will start by saying that this computer is owned by some one that smokes next to there pc case, this in turn will also increase the amount of dust that builds up over time inside the computer, this is because the smoke from the cigarette goes inside the computer system and then makes a nice layer of sticky nicotine inside the case, then the dust sticks to this nicotine. Please note not smoking next to a computer doesn’t mean there will be no build up of dust.
Ok here is some picture of the inside of the computer case
Now here is the CPU heat sink and fan, before I started cleaning the computer I got it running up to full temperature with no programs running, this temperature was:
CPU = 48c
And the fan speed on the heat sink was
Fan speed = 2857rpm
Now here are the images of the CPU heat sink and fan
now this is bad 
After a really bad and dirty cleaning job of 3 hours, yes
3 hours I them placed everything back inside the computer system, here are some images of the system now clean.
Yes believe me or not it is the same system.
Now after running the system and getting it up to full working temperature and with know other programs running, I checked the temperature and CPU heat sink fan speed, this you will think I am lying (I AM NOT)
CPU TMP = 39c Fan Speed = 3276rpm
This was amazing to see that all that dust made the temperature on the CPU run at a whopping extra 9c and the CPU heat sink fan run at a slower speed because it was getting restricted but after cleaning this increased the fan speed of an extra 419rpm.
This in tern will increase the speed of the computer and the response time on running programs, also this will increase the life span of the central processing unit (CPU) and the CPU heat sink fan.
How often do you clean your computer?