Question: I am curious how spy++ Finder Tool finds out the window handle for the window over which the mouse is. 
Is there any WIN32 function for getting the handle of the topmost window that occupies a certain pixel on the display?

Answer: 
There is a [WindowFromPoint()](http://msdn.microsoft.com/en-us/library/ms633558%28VS.85%29.aspx) function.
[`WindowFromPoint`](http://msdn.microsoft.com/library/ms633558(VS.85)) or [`ChildWindowFromPoint`](http://msdn.microsoft.com/library/ms632676(VS.85)) API functions.