This is a VERY simple fix when you start WMP 12 it starts Maximised but you click play now screen it always started minimised well after searchin a lot of places trying to find an answer i found a very & i mean simple
Forcing a window to open maximized...
Hold down <Ctrl+Alt> while dragging the window to fill the screen,
do not release the keys, then close the window then release the keys.
It may work on other application windows i have only done it on WMP 12