There is sometimes a fungus that can develop on the lens of a very old camera or one that has not been cleaned very often. This is seen as a dull substance on the glass that is not removed with a glass cleaner. Use a light coating of mineral oil and let stand overnight, then wash with Windex or other glass cleaner until all residue is gone.
Dust in a camera can be a real problem. Canned air, available at most computer stores for cleaning keyboards and computers is a low cost option to dust out the camera on a regular basis. If you do this routinely and keep the camera in a case when not in use dust will not become a problem.
It is better to take your camera into a camera repair professional before taking it apart if you are not comfortable with the process. When you take the camera apart you can often cause more damage or even lose the small pieces that are needed to correctly re-assemble the camera. In the long run replacing these parts will add to the bill.
