make two movie clips, one of just a vertical line, and one of just a horizontal line. drag these onto the stage and call them 'vertline' and 'horizline'.
make a new layer in the main scene and make two keyframes next to each other (e.g. frame 1 and frame 2). open the actionscript window for frame 1 and put:
Call(2);
now open the actionscript window for frame 2 and put this in:
vertline_x = _root._xmouse;
horizline_y = _root._ymouse;
GoToAndPlay(1);
i haven't tested this but it looks right..should work in flash 5.