Sensing the start of a personal computer revolution, Bill Gates dropped out of Harvard University in 1975 to start Microsoft Corp and pursue a vision of a computer on every desk and in every home.
Three decades later, Gates is set to step down on Friday from what is now the world's largest software company to work full-time at the charitable organisation - the Bill & Melinda Gates Foundation - built by his vast fortune.
No longer the world's richest man - he has been topped by investor Warren Buffett and Mexico's telecoms tycoon Carlos Slim - Gates says great wealth brings with it great responsibility.
The 52-year-old, whose boyish looks seem at odds with his graying hair, will leave behind a life's work developing software to devote energy to finding new vaccines or to micro-finance projects in the developing world.
As Microsoft's biggest shareholder, Gates will remain chairman and work on special technology projects. His 8.7 percent stake in Microsoft is worth about $23 billion.
Gates first programmed a computer at 13, creating a class scheduling system for his Seattle high school. As he gained more experience, he realised the potential that software held to change how humans worked, played and communicated. "When I was 19, I caught sight of the future inside that Gates met Allen, a student two years his senior who shared his fascination with computers.
"Of course, in those days we were just goofing around, or so we thought," Gates recalled in "The Road Ahead." During his two years at Harvard, Gates devoted much of his time to programming marathons and all-night poker sessions before dropping out to work on software for the Altair, a clunky desktop computer that cost $400 in kit form.
Also at Harvard, Gates became friends with an ebullient Detroit native who shared his love of math and cynical humour. Gates eventually talked that classmate, current CEO Steve Ballmer, into leaving business school to join Microsoft. Gates dropped out of Harvard and relocated with Allen to Albuquerque, New Mexico, where they established Microsoft.
Their big break came in 1980 when Gates and his carelessly dressed young colleagues signed an agreement to build the operating system that became known as MS-DOS for International Business Machines Corp's new personal computer.
In a critical blunder by IBM, Microsoft was allowed to license the operating system to others, spawning an industry of "IBM-compatible" machines dependent on Microsoft software.
"His legacy has to be as one of the shrewdest businessmen and technologist of the 20th century," said Michael Cusumano, a professor at the Massachusetts Institute of Technology's Sloan School of Management.
Microsoft went public in 1986 in one of the most celebrated offerings of its time. By the next year, the soaring stock made Gates, at age 31, the youngest self-made billionaire.