During one holiday to the Caribbean with his wife Melinda, Bill Gates took seven volumes of World Health Organisation data to read on the beach.

When concentrating, Mr Gates is known to sit, arms wrapped around himself, in something that resembles the foetal position, and rock backwards and forwards in his chair. He wears a uniform of Gap-style casual clothes, drinks Diet Coke and is called “Trey” by his family and close friends - an old family nickname.

Such is the rather intense man who, from yesterday, will devote most of his time to running the Bill and Melinda Gates Foundation - the charitable trust that holds about $37.5 billion (£19 billion), with about $16.5 billion already spent. The foundation is run - it appears with equanimity - by the couple, who married in 1994, and is focused on “innovation in health and learning to the global community”. It employs almost 600 people.

The fund is sliced into two parts. The first helps to reform the US state school system, investing in technology and libraries for American children. The other, which is financed by 60 per cent of the fund, is aimed at fighting disease in the developing world.

Observers of the couple claim that Melinda offsets her husband's intensity and intellectual arrogance, and is perceived to be a purposeful and openly compassionate person.

Choosing a challenge as technical and complex as global healthcare may prove to be just as complex as gaining control of the home computer market. Mr Gates approaches the task of who and what to invest in, what targets to chase, by crunching data.

He has sought to redefine philanthropy: it is not, he argues, the retirement hobby of the rich and restless, but about identifying the best investments and outcomes for saving lives.

The billionaire has spent a lifetime learning how to identify such investments since founding Microsoft in 1975 with Paul Allen, his old school friend. The pair started writing code, formed Microsoft and began developing an operating system - called QDOS (Quick and Dirty Operating System) and sold it to IBM. As the sale of home computers took off, so did demand for the Microsoft software.

Under the singular leadership of Mr Gates, the company began to have run-ins with anti-monopoly authorities, resulting in a court battle that began in 1998 in the US for bundling its Explorer internet search engine into the package of its other products.

Rise of a dropout

1975 Bill Gates drops out of Harvard and starts Microsoft with a friend, Paul Allen. They write Basic, a computer programming language, Microsoft's first product

1980 Microsoft creates operating system for first IBM PC. Microsoft buys the software for $50,000 from another company and calls it MS-DOS. Steve Ballmer joins Microsoft

1983 Introduces Word word-processor

1985 First version of Windows

1989 First version of Office business software

1995 Windows 95 and Internet Explorer 2.0

1998 Ballmer president

2000 Gates steps aside as chief executive in favour of Ballmer, but remains chairman and becomes chief software architect. Launches Windows 2000

2001 Windows XP

2005 Microsoft Xbox 360 console2007 Releases long-delayed Windows Vista and Office 2007

Jan 2008 Microsoft makes unsolicited $44.6 billion bid for Yahoo!. Eventually walks away after Yahoo! rejects raised offer of $47.5 billion

Feb 2008 The EU imposes record €899 million fine on Microsoft for using high prices to discourage software competition

June 2008 Gates steps down from full-time role in company, but remains chairman

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