XP strange in that. Do (sorry I do that in a turkish windows. may be translate mistakes):
-create a shortcut for your dos fpd26 program.
-run with click.
-properties.
-full screen (don't forget select 25 lines).
-select second option (like do this settings default for next)
-exit application.

now if you reenter you will see your application will in a window.
-don't enter application.
-properties from shortcut.
-select full screen.

now your application will work fullscreen 25 lines everytime :)