A window defined by an add-on program is displayed to a user of an application program in a computer system. The add-on program detects the display to the user of an application window created by the application program. In response to this detection, the add-on program displays an add-on window to the user. The add-on window may enable the user to initiate new functions in the computer system, and the add-on window may be a child window (which is defined by the add-on program) that is embedded in a parent window displayed by the application program.