A display screen magnifier is controllable by a user to magnify selected portions of a desktop including windows open on the screen. A moveable magnifier frame is provided, within which a magnified image is presented. The magnified image corresponds to screen content located within a magnification region underlying the magnifier frame, magnified to a set level of magnification. The magnifier is controllable through use of a user input device. At least one operational parameter of the magnifier can be adjusted, without interaction with on-screen objects and without taking focus away from the desktop or any windows open on the screen. In a particular embodiment of the invention, on-the-fly controllability of the magnifier includes its activation and deactivation, adjustment of the magnification level, and adjustment of the size of the magnifier frame.