1993: Windows NT 3.1
Screenshot courtesy of GUIdebook.
Windows NT 3.1, released in July 1993, was built for businesses rather than consumers and was designed to be more secure and stable. It used a 32-bit rather than a 16-bit architecture. Version 3.1 was the first release of the NT operating system; earlier version numbers were skipped so that the numbering for this business OS would match that of the consumer OS, Windows 3.1. NT required an 80386 CPU, 12MB of RAM (16MB was recommended) and 90MB of free hard disk space.
The enterprise-oriented operating system went through three more releases -- Windows NT 3.5 in 1994, Windows NT 3.51 in 1995 and Windows NT 4.0 in 1996 -- before moving to Microsoft's year-based version numbering convention with Windows 2000.