The condition of a window or full screen of an Excel spreadsheet is saved as part of the particular file Hence, when it is opened again, it will open as a window or full screen, as it was saved. So, Excel cannot simply default to one or the other. However, if your mouse has a wheel, you may click on Tools, Options, General, and check the "Zoom on roll with IntelMouse. Now, whenever a spreadseet opens as a window, you can very easily zoom you sheet to full screen. Does this solve your problem?