Squeeking floorboards--what is the best way to prevent /remedy this?

A squeaky floor is caused by nails. The nails used to secure the sub-floor and or the unerlayment are experiencing movement from the wood itself. I would need to know if the squeak is under carpet, linoleum, or other surface. There is a kit you can buy to take care of most squeaks. For carpeting they make a tool that can place a screw in the floor without causing any damage to the carpet. Can you elaborate where the squeak is?