Window message sent to my topmost form/window when the taskbar/start menu is over it?

Question

I have a C# Windows Forms program, in Windows 7, but I think it applies to any Windows program.

The main form of my program covers the entire screen, and has the Topmost property set. However, when I click the Start key to open the Start menu for example, the taskbar and Start Menu are displayed, on top of my Always on Top form.

Is there a window message (WM_) that is sent to my form/window when this happens and my form/window is no longer the topmost window (or on top of the Z-order)? I'd like to catch it using WndProc.

Thank you