The term 'precursor to 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 first widely used operating system for IBM-compatible PCs and was the predecessor to the Windows operating system. DOS, or Disk Operating System, was the original operating system developed by Microsoft in the late 1970s. It was the foundation of the MSDOS operating system and was the first operating system to be used on IBM-compatible PCs. Both MSDOS and DOS were the precursors to the Windows operating system, which was released in 1985.