Everything is displayed in a window, event the screen savers.
For screen savers the window is full size and has no borders or title bar.
You can do the same thing with the c# forms.
Set the form's FormBorderStyle property to None.
Note: Make sure you have a way to close the form before you test this as you will loose the X button at the top.
That is a new question and you should realy start a new thread.
Essencially, yes it is possible. However, if you mean to put windows over other applications then finding the position of the other app windows is not easy.
Start a new thread and maybe someone will advise you.