1: Microsoft DOS
In 1991, PCs predominantly ran on Microsoft's DOS Operating System. Unlike today, where the GUI (Graphical User Interface) is taken for granted in PCs for the OS and applications, the GUI and the OS were separate products. Extremely primitive by today's standards, DOS was a 16-Bit character-mode OS and had no built-in multitasking capabilities. It also used an unjournaled 16-bit directory-based filesystem, FAT, which was used on both floppy and hard disks and had an 11 character limit for filenames, hence the "8.3" file format with names such as AUTOEXEC.BAT.
In addition to Microsoft's DOS, IBM had it's own version, PC-DOS, that ran specifically on its PS/2 personal computers. Digital Research, which pioneered in the late 1970's with the forerunner to DOS, CP/M, also released its own DOS-compatible OS, DR-DOS, and eventually ended being owned by Novell and later, SCO.