> The AARD code was a segment of code in a beta release of Microsoft Windows 3.1 that would determine whether Windows was running on MS-DOS or PC DOS, rather than a competing workalike such as DR-DOS, and would result in a cryptic error message in the latter case. This XOR-encrypted, self-modifying, and deliberately obfuscated machine code used a variety of undocumented DOS structures and functions to perform its work, and appeared in the installer, WIN.COM, and several other executables in the OS.[1]

https://www.theregister.co.uk/1999/11/05/how_ms_played_the_i...

> Microsoft had several methods of detecting and sabotaging the use of DR-DOS with Windows, one incorporated into "Bambi", the code name that Microsoft used for its disk cache utility (SMARTDRV) that detected DR-DOS and refused to load it for Windows 3.1. The AARD code trickery is well-known, but Caldera is now pursuing four other deliberate incompatibilities. One of them was a version check in XMS in the Windows 3.1 setup program which produced the message: "The XMS driver you have installed is not compatible with Windows. You must remove it before setup can successfully install Windows."