I need to open the housing of a us robotics 56k external modem, but there is not any screw visible and I see it is risky to break the box if trying to separate the upper and lower parts of the box by hand. Is there any idea how to do it?
Vangelis

Try it with some small screwdrivers and/or knives. You might crack the case a little bit, but that can be glued together again. Just do it gently, it should open.
Replacement internal modems (better than the one you have), are quite cheap.
Opening it up will not do anything for you, unless you are an electronics specialist.