Microsoft has used Windows as its flagship software product since 1985.
Microsoft released Windows 1.0 on November 20, 1985. It began as a graphical operating environment that ran on top of MS-DOS, rather than as a completely independent operating system. Later releases, especially Windows 3.0, Windows 95, and Windows XP, helped make personal computers common in homes and offices.
The company was founded by Bill Gates and Paul Allen in 1975. Its business later expanded from PC software into enterprise computing, cloud services, gaming, hardware, advertising, and professional software. Azure is now a major part of its cloud business, while products such as Microsoft 365 and Xbox broaden its reach beyond Windows.
A common mix-up is treating Windows and Microsoft as interchangeable names. Windows is a Microsoft product; Microsoft is the corporation that develops and sells it. Market-cap rankings measure the value investors place on Microsoft as a whole, not merely on Windows.