Qt is a multi platform library for C++ (bindings other programming languages also available) which covers a lof of fields like GUI, string handling, network, multi-media. In my mind, for real C++ programmers who write for – but not only for – Windows Qt is currently the top choice.
In 2009 I have made a FOSSLC boot camp (or better the second half of a C++ boot camp) with an introducton to Qt.
The main aim was to show the principle behind Qt and to make a first sample application. Here are the material to this lecture: