You’re posting on the wrong forum. This forum is about QNX, a completely different operating system from Windows, with a completely different API for GUI programing. So you’re asking the wrong crowd.
Having said that, you probably DON’T want to use the Win32 API. It is old, difficult to work with, and essentially deprecated in favor of newer technologies. For better or worse, virtually no one programs directly in the Win32 API any more.
For straight C, the most obvious choice for GUI is GTK+, although it’s a little tricky on Windows. You could also look at IUP, though I don’t know much about it. There’s even wxC, although that doesn’t look like an easy option.