A dirty computer has a shorter life span.
When you clean your pc on a regular basis you will help improve performance and increase the life of your system.
Computers attract dust, so it is important to try and place it on a flat hard surface. If the only option is to have it on a carpeted floor, you should get a mat or board on which to place it. This will help reduce how much dust gets sucked into the computer by the cooling fans.
Turn off the computer, open the computer case and look inside. If there is a lot of dust build up on the inside, then you should buy some canned air or get an air compressor and blow out all of that dust. The dust build up causes the computer to get hot. Too much heat can effect performance, shut down or burn out the computer.
If you are keeping the computer in a small space or cupboard, be sure that the space is well ventilated and has good airflow, especially if it has a door.