Embedded system engineer's guide to GUI
The very simplest way to generate a GUI application with a range of simple dialogs is to use a small tool called Zenity (another version is called Gdialog). 

It is actually a small program which you can run from the command line, or shell script or batch file. Or however you invoke an external command from your C program. When it exits, it gives a return code and a string on stdout : this string is the output of that dialog : filename to open, the list item selected, the edited text file, whatever.

It won't do complex things such as graphics but for simple GUI apps it works really well and it's much simpler to develop an app : test each dialog directly from the command line until it does what you want.

I've only used it on Linux but it's such a handy little tool, it would surprise me if there wasn't a Windows port available.