What I do is build a temporary partly wall in the room (from an opposite wall to about halfway the room). Center this new wall in the room. Now it is possible to center the window in this half room. The temporary wall can now be removed. When you want more windows this division can be repeated to divide the room in 4 parts. Also remember it is possible to select multiple objects and center the entire group (you often find that centering things just doesn't look right, so place two windows where they look about right, select them both and center).