Microsoft Windows

The Windows 1945 is the operative system of the Führerbunker.

Having only appeared in a parody by hitlerrantsparodies it is portrayed as extremely slow to the point of taking two weeks to install, 5 hours to start up and 4 bytes of RAM to render the cursor. Hitler even referred to it as shit compared to Windows 1942.

Trivia
In Tobbs96's Ranting Club (part II) it is mentioned the existence of a System which might probably be the same.