Final answer:
The operating system used by Microsoft is Windows, which faced antitrust litigation from the DOJ for bundling Internet Explorer, leading to accusations of unfair competition, anti-competitive behavior, and planned obsolescence. Despite the legal battles, Windows remains dominant in the market.
Step-by-step explanation:
The operating system used by Microsoft is Windows. This system has been bundled with the company's web browser, Internet Explorer, resulting in antitrust litigation from the mid-1990s until 2004. The U.S. Department of Justice (DOJ) prosecuted Microsoft, claiming that the inclusion of Internet Explorer as the default browser was an unfair competition practice that harmed competitors like Netscape Navigator. The DOJ contended that since Windows operated on the vast majority of personal computers, competitors could not gain market traction. In addition, there were concerns about planned obsolescence in Microsoft Windows, where newer operating systems would quickly make previous versions obsolete as they weren't able to open the new files created by updated software, compelling users to upgrade.
The antitrust regulators argued that Microsoft's actions extended beyond profiting from their dominant position in the operating systems market and constituted anti-competitive behavior. This included threats to computer manufacturers that if they didn't omit competing software, notably Netscape's Internet browser, Microsoft would not sell them its own operating system. The regulators accused Microsoft of predatory pricing, by offering additional software products for free with Windows, aiming to oust other software makers from the market. Despite the accusations and subsequent legal challenges, Microsoft has maintained a significant presence in the operating systems market, with their Windows system present on over 90% of the most commonly sold personal computers as of 2013.