November 20, 1985 – Just four months later (Phew! 1985 was a busy year!) Microsoft joined the party with Windows 1.0. The user interface consisted of tiled windows (overlapping windows were added in Windows 2.0, for which Apple sued Microsoft.) Properly written MS-DOS software could run in a multitasking environment. This gave Microsoft a bit of an advantage in the number of software packages available for their fledgling graphical environment.
Fun Video: Watch ex-CEO Steve Ballmer pitch Windows 1.0.