WINVER not defined, defaulting to…

WINVER is a symbol that your Windows application is suppose to define to declare the oldest version of the OS that your application  is designed to run on.  If you do not define it, you will get a warning and it will default to the OS on which you are building.

ox501 is the value to use for  Windows  XP.   You can define it in stdafx.h or in the preprocessor settings of your project.

Read more about it at here.