WINDIR Environment Variable Set When Running Windows

This article was previously published under Q65662

Microsoft Windows version 3.0 sets the WINDIR environment variable whenever any program, including a MS-DOS program, is run under it. WINDIR is set to the directory where WIN.COM is found. This is usually C:\WINDOWS. The WINDIR variable is removed from the MS-DOS environment when you exit Windows.

As a result, MS-DOS applications can detect whether they are running under Windows. If WINDIR is set to a valid name, the application is running under Windows.