Check the settings for your monitor; you can do this by clicking the Windows Start button, then click Settings and Control Panel. In the control panel, double-click Display. This opens a window describing your display screen.
In this new window, click the Settings tab. This shows the settings for using your screen. MainBoss needs a Desktop Area (resolution) of 800x600 pixels or higher. If you use a resolution of exactly 800x600, you should also specify Small Fonts under Font Size. Making these changes should fix your problem.