Old PC Software is a term used to describe software that was developed for computers running the MS-DOS operating system. MS-DOS, which stands for Microsoft Disk Operating System, was a command line-based operating system released by Microsoft in 1981. It was the most widely used operating system on personal computers until the mid-1990s when Windows 95 was released. MS-DOS was the foundation of the Windows operating system and was used to run many popular programs such as WordPerfect, Lotus 1-2-3, and Microsoft Office.