if you use Tv mode, the camera should set the aperture to the smallest according to the exposure metering. if you get over exposed picture, that means you have reached the minimum aperture your lens can handle. in this case you should notice your aperture value flashing (or something to that effect) which is the way the camera tells you that it cannot achieve correct exposure. gotta pay attention to all those flashing things in your view finder you know ;p
If you used Tv and still get overexposure, it means that the camera has probably selected the smallest aperture but the scene is still too bright. Like what "jnet6" and "JediForce4ever" mentioned, use a ND filter or use a lower ISO film or select a lower ISO in the case of digital.