Reading and Writing Files
In windows you have a few options as to how you want to access files. You can use the old io.h open()/read()/write(), you can use stdio.h fopen()/fread()/fwrite(), and if you are in C++ use can use iostreams.
However in windows all of these method ultimately call the Win32 API functions, which are what I will use here. If you are already comfortable using file IO with another method it should be fairly easy to pick up, or if you want simply use your method of choice to access files.
To open files, you can use OpenFile() or CreateFile(). MS recommends using only CreateFile() as OpenFile() is now "obsolete". CreateFile() is a much more versatile function and provides a great deal of control over the way you open files.
Reading
Say for example you have allowed the user to select a file using GetOpenFileName()...