The term 'forerunner of Windows' can be summarized by two crossword solutions: MSDOS (5 letters) and DOS (3 letters). MSDOS stands for Microsoft Disk Operating System and was the first operating system released by Microsoft in 1981. It was the predecessor to the more popular DOS, which stands for Disk Operating System. DOS was released in 1983 and was the first operating system to be widely used on personal computers. Both MSDOS and DOS were the forerunners of the Windows operating system, which was released in 1985.