Windows Precusor is a term used to refer to the operating system that preceded Windows. MS-DOS (5 letters) and DOS (3 letters) are two of the most popular versions of this operating system. MS-DOS was released in 1981 and was the first operating system to be widely used on personal computers. DOS was released in 1983 and was the first version of MS-DOS to include a graphical user interface. Both versions of the operating system were popular until the release of Windows in 1985.