I have a fullscreen window with the Topmost property set to true. Whenever an application changes its WindowState property, my window is automatically minimized even though
If this is a kiosk application and you are the only program running on the computer then fine. Otherwise this is a really bad idea and that's why there is not a documented way of doing it. As Raymund Chen says this kind of question should be followed with the thought experiment "what if two programs did this?":
http://blogs.msdn.com/oldnewthing/archive/2005/06/07/426294.aspx
You may want to investigate "Kiosk Mode".
Perhaps this is what you want: Windows Form in Full Screen "Kiosk Mode".
Otherwise I'll second Henk's recomendation.