Bill Gates, currently the world's third richest man, will step down today as chairman of Microsoft signalling the end of an era that spans more than three decades.

Sometimes derided as a geek who relied on monopolistic practices to carve out his multi-billion dollar empire, Gates is arguably the only leading figure in the IT industry who can lay claim to being the 20th century's most influential entrepreneur.

Gates co-founded Microsoft with Paul Allen in Albuquerque in 1975 after dropping out of Harvard, with a mission to get a PC in every office and home.

Some 33 years later, more than 90 per cent of the world's desktop PCs run on Windows and virtually all of the world's word processing documents and spreadsheets are created using Microsoft software.

In 1995 Gates officially became the world's richest man with an estimated fortune of more than $12.9bn, but then endured several years of gruelling uncertainty in a bruising antitrust case with the US government that threatened the break up of the company.

In June 2006, he announced his intention to step down as Microsoft chairman by July 2008 to devote his life to full-time philanthropy through the Global Health and Development Program run by the Bill and Melinda Gates Foundation.

Gates will stay on as Microsoft chairman in a ceremonial capacity, but his day-to-day efforts will now focus on the eradication of malaria and the development of an Aids vaccine, among other projects in the developing world.

Gates leaves behind him a triumvirate of chief executive Steve Ballmer, chief research and strategy officer Craig Mundie and chief software architect Ray Ozzie.

The trio are charged with fending off the growing popularity of open source software and competition from relative newcomers such as Google.