Assuming that you dropped the screw into the top of the main control unit you should be able to open it and remove the screw, Unplug it from AC before you start.
Chances of it causing a problem aren't real high but it is possible. Are you feeling lucky?
as long as you dont power the pc up or plug it into the wall. dropping a screw wont hurt you as long as you find it. look under the mb and shake it out.
Assuming that you dropped the screw into the top of the main control unit you should be able to open it and remove the screw, Unplug it from AC before you start.
Chances of it causing a problem aren't real high but it is possible. Are you feeling lucky?