Everything, including the GUI, is part of the open source code.
Most of the dialogs in Bitcoin-Qt are created using Qt designer (.ui xml files in src/qt/forms), which means that you can edit them using a graphical interface from the Qt Creator IDE. This is freely downloadable.
Although Qt creator is available for windows, building Bitcoin-Qt on Windows is pretty complicated due to the dependencies (see
https://bitcointalk.org/index.php?topic=149479.0), if you want to play around with Bitcoin development I recommend using a Ubuntu VM and installing the Qt Creator package.