Aside note...

Dealing with legacy projects is not an easy task and requires some knowledge and experience. Simply getting tweaks from old sources is not enough. This particular case requires at least basic understanding of Windows concepts.

Especially for GUI part, I kindly recommend Win32 Programming by Brent E. Rector and Joseph M. Newcomer. It's an old book, but still
actual and it was more than a "bible" for me.

Then, programming with MFC or OWL, which are just C++ wrappers around plain Windows API, is like a walking in the park.