I'll explain...
first, I think you added the lines in the shortcut of MaximizeWindow...
remember if you did set your game resolution at 1024 x 768, the Windows Desktop resolution should be at 1024 x 768
now right click on MaximizeWindow shortcut and click properties, in "target" box, the last line should be this : \MaximizedWindow.exe"
what you need to do is this \MaximizedWindow.exe" 10 1280 1024
10 : is the timeout (the game won't maximized until the dos box disappears)
1280 and 1024 are the resolution set to make the windowed disappear (it should be set a one res higher)
so if your game res is 1280 x 1024, your Desktop res should be the same, and you will write the next big res in the MaximizedWindow shortcut...
to launch the game, first click on the game shortcut, the menu should appear, go to options and select Windowed mode...
when you want to click on play, launch MaximizedWindow shortcut and then click play, depending on the timeout you gave the program to close the window mode will disappear.
of course every time you need to launch Fallout 3, you should launch MaximizedWindow before it, it could be a pain in the *** but it helps
Good Luck