An archaic operating system from the early 1990s.
Unlike modern PCs and MACs, it didn't have a START menu or toolbar; it had a program manager application. It was one of the first GUI Windows platforms, an upgrade of Windows 3.0.
3.1 was the first system I ever worked with, waaay back when I was four or five. 3.1 was a good machine, capable of handling any game from Reader Rabbit to the Oregon Trail (CD).
Then we upgraded to Windows 95, which had a better interface.
But Windows 3.1 will be dear to me forever, for it was my first computer OS ever.