I am including a third party header and source file into my project.
At the top of the header there is this:
#if defined(WIN32) || defined(WIN16) #i
Check your includes. I am guessing that the third party header is included prior to the windows.h. So, in your main.cpp or equal it should be
#include // this will also include windefs.h #include
and not the other way around.
Hope that helps.