This article shows that dust can cause your desktop computer to slow down.
A customer brought in a Dell E520 desktop computer. He said it ran well for about an hour but then slowed right down.
The reason was heat, the computer processor and motherboard was getting warm because cool air was unable to circulate the computer case freely. To protect itself a processor will reduce its power to lower the heat produced. This then slows down the computer.
From the two photos of the same Dell E520 computer above you can see a clue to the amount of dust to expect, there is dust in the corners, on the case, on the graphics card and on the motherboard.
The black plastic box with the 6 holes and a clip on covers the heatsink that cools the processor. To remove it you need to use a long positive screw driver to undo the two screws either side. Once they are undone use a little pressure to lift the screw end and it should come out revealing the dust causing the actual problem.
The photo above shows dust blocking the cooling fins of the heatsink. This dust is stopping cool air from passing through the fins therefore trapping the heat.
The fix requires a paint brush, a vacuum cleaner with pipe attachment, paper towels to wipe the dried thermal compound from the processor and heatsink and fresh thermal compound.
Using the paint brush gently brush all the dust so that the vacuum cleaner can pick it up. The vacuum should pull all the dust out of the heatsink but a gentle tap may help loosen some of it. Once all dust is removed clean the old, dried thermal paste from the processor and the heatsink. Reapply thermal paste and put the computer back together.