Quote Originally Posted by wysota View Post
Quoting the docs:

This feature is available on Embedded Linux, Mac OS X, Windows, and X11 platforms that support the Composite extension.

This feature is not available on Windows CE.

Note that under X11 you need to have a composite manager running, and the X11 specific _NET_WM_WINDOW_OPACITY atom needs to be supported by the window manager you are using.
Yes, I'm aware. I'm running Windows 7.