The default desktop window is "SHELLDLL_DefView", under the "Progman" window, you write a program to detect, find and click to display the desktop, the desktop window went down "WorkerW" window. In this case, any program window to activate this special status is gone, back to the table under "Progman" window.
In fact, the system displays the desktop features, not all applications on the desktop window is hidden or minimized, but a special status, "WorkerW" are hidden by default, when you want to display the desktop will be displayed, and window Z the order went to the top, then "SHELLDLL_DefView" Desktop parent window by the "Progman" changed to "WorkerW", then the application may also be in a particular state. So you use IsWindowVisble, IsIconic and other functions that can not be detected, unless the point show the desktop, and activate any of a window.