the #if !defined .... #define ....
simply tells the compiler not to define the header file again if it's
already defined once

WIN32_LEAN_AND_MEAN just loads the basic headers that one
needs to create a window without all the extra junk that you
probably wouldn't use. I'm not sure where exactly the line is
drawn as to what is loaded and what isn't..... CornedBee would
probably (definitely) know

i'm not sure what #pragma once means however.....CornedBee
again