SAN FRANCISCO, Feb 4 (Reuters) - Satya Nadella's elevation to CEO and co-founder Bill Gates' decision to relinquish his chairmanship at Microsoft Corp marks a changing of the guard at a 39-year-old company that fueled the PC revolution but is now struggling to establish its long-term identity.
Here are some highlights in the four-decade journey of the world's largest software maker:
1975 - A 19-year-old Gates drops out of Harvard University and goes on to found Microsoft with childhood friend Paul Allen. They make their first product - a BASIC programming code interpreter for the Altair 8800 microcomputer.
1979 - With sales topping $1 million at the end of 1978, the company moves to Bellevue, Washington, from Albuquerque, New Mexico.
1980 - In June, Gates recruits former Harvard classmate Steve Ballmer to become the company's first business manager.
1981 - In August, Microsoft releases its new operating software Microsoft Disk Operating System (MS-DOS). It begins running on IBM personal computers - a seminal moment for the company's future domination of personal computing systems.
1983 - Microsoft announces its new software "Windows," which aims to enhance the MS-DOS interface with visual features.
1985 - After spending two years in development, "Windows 1.0" begins to ship. Moving on from MS-DOS commands, which some users found challenging to master, the system used simple clicks of a mouse to work through tasks on screens or "windows."
1986 - Microsoft moves its corporate headquarters to Redmond, Washington. It goes public at $21 per share and raises about $60 million.
1988 - With the arrival of Windows 2.0 in 1987, computers start becoming more commonplace in the office. Microsoft becomes the largest PC software company based on global sales.
1990 to 1995 - Windows 3.0 is launched in 1990 and five years later, Windows 95 is released and sales surpass 1 million copies in four days. PC sales begin to explode as computers make their way into homes, schools and business, kicking off the "Windows era."
1995 - With the advent of the Internet, Microsoft introduces its Web browser "Internet Explorer."
1998 - Windows 98, the first consumer-focused version of Windows, is released. That same year, the U.S. Department of Justice files antitrust charges against Microsoft relating to bundling of its programs into its operating systems. U.S. regulators accuse the company of using its dominance in software and monopolistic practices to drive competitors out of business.
2000 - Ballmer succeeds Gates as CEO in January. The two met as students at Harvard, when Gates lived down the hall from Ballmer.
2001 - Microsoft reaches a settlement with the Department of Justice in the 1998 antitrust case. In November, Microsoft enters the gaming market with the North American release of its Xbox gaming console.
2005 - The next-generation Xbox 360 goes on sale in November.
2006 - Microsoft launches the Zune portable music player in November, the first Microsoft-designed device to compete in a market dominated by Apple Inc's iPod. The music player fails to take off and Microsoft discontinues it by mid-2012.
2007 - In January, Microsoft unveils the widely panned Windows Vista, the company's least-popular operating system.
2009 - Microsoft revamps its search engine to counter Google's dominance in Web search and advertising. In May, Ballmer reveals the new engine, dubbed "Bing." It remains a distant runner-up to Google today.
2010 - Microsoft releases its Windows Phone operating system for mobiles, trying to woo consumers away from Apple's iPhone and Google Inc's Android devices. The Windows ecosystem remains a distant third to Apple's iOS and Google's Android.
2012 - Microsoft, hoping for a hit, launches Surface tablets and a Windows 8 operating system that uses touch commands. The devices have not gained much share in a tablet market dominated by Apple's iPad, Amazon Inc's Kindle and Samsung devices.
2013 - On Aug. 23, Ballmer announces he will step down within 12 months, surprising industry-watchers.
Feb. 4, 2014 - Microsoft's board unanimously picks Nadella, 46, an Indian-born executive who led the creation of Microsoft's Internet-based or "cloud" computing services. He is only the company's third CEO in 39 years.