SEATTLE (Reuters) - 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 organization -- 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 realized the potential that software held to change how humans worked, played and communicated.

"When I was 19, I caught sight of the future and based my career on what I saw. I turned out to have been right," Gates wrote in his 1995 book "The Road Ahead."

Gates realized at an early stage of the PC revolution that software would be more important than hardware. Working with boyhood friend Paul Allen, Gates founded Microsoft, naming the company for its mission of providing microcomputer software.