AARD code
The AARD code was a compatibility check in the installer for Microsoft Windows 3.1. The checker would cause the installation to fail on non-Microsoft operating systems. This became an issue with the popular DR DOS operating system, and some accused Microsoft of attempting to stop DR DOS while promoting their own DOS operating system, MS DOS.
At the time, DR DOS was owned by Digital Research, who released a patch to allow the installation on DR DOS. Microsoft disabled the AARD code, however, for the final release of Windows 3.1.
The name "AARD" is derived from Microsoft programmer Aaron Reynolds. (see [1])