a basic inside-case clean is usually a good idea (exept laptops, unless you know your way around PCB safety and handling)
If it is a desktop, still under warranty, opening the case might not be the best idea.
if it is outside the grasp of warranty, i would recommend the case clean, a slightly wet cloth (such as a single spray of water from a spray bottle) should be good to clean ONLY the fan or case. NOT the circuit boards.
another reason for the problem could just be a loose power wire from the Mother Board to the fan. Solution would be to plug it back in, but caution must be taken
Remember that static discharge does ruin computers, so make sure you ground yourself to the computer. The most simple way to do this is to make sure your skin is in contact with an exposed metal part of the case (usually the back of the case or the floor of the interior)
However, i would not recommend taking caution, as doing the wrong thing inside the case could cause more problems.
