Until the corporate image people got hold of him, Mr Bill Gates or William H. Gates III was the epitome of the computer nerd.

Writer Mr Robert X. Cringely, the pen-name of the man who wrote and presented the popular television series Triumph of the Nerds, has described the earlier version of Gates as famously dishevelled, unkempt and oily-haired.

Nowadays Mr Gates, born in 1955 to a Seattle attorney father and schoolteacher mother, appears in expensive jackets and shoes and sports a stylish haircut and glasses. With a personal fortune of somewhere around $60 billion, the unhip geek of yore can afford to buy David Bowie and Rolling Stones songs as Microsoft corporate ditties.

If style and grace never seemed to mark Mr Gates out as the future richest man in the world, he was reportedly an extremely competitive child, voluble and strong-willed, so much so that at age six his parents put him in counselling. Computers were an early interest, and he began programming them at 13.

His initial claim to fame was crafting a version of the BASIC programming language that allowed people to write programs for the first hobbyist computer, the MITS Altair, which could be built from a kit.

In 1975, he dropped out of Harvard in his third year to set up Microsoft with his old friend Paul Allen.

They purchased the rights to an operating system known as QDOS - Quick and Dirty Operating System - from a Seattle-area company and released it as MSDOS - the operating system which would become the dominant operating system in the personal computer market. In response to the release of the Apple Macintosh, the first personal computer with a graphics rather than text-based interface, Mr Gates led Microsoft to create the Windows operating system, now used on more than 90 per cent of personal computers worldwide.

Gates is formidably intelligent and has been described as a ruthless businessman. He has spearheaded Microsoft's drive to acquire companies which have developed technologies he feels would be useful to Microsoft, leading to accusations that Microsoft itself buys up innovation rather than creating it itself.

Mr Gates married his secretary, Ms Melinda French, in 1994 and the couple now have a small daughter. His most distinctive and well-known personal habit is rocking himself back and forth vigorously while thinking.

His many interests outside Microsoft include a stake in satellite communications company Teledesic, and ownership of visual information and digital archive company Corbis, which has the rights to material such as the paintings in the British National Gallery.

One of the world's richest men, Mr Gates has also set his sights on becoming a well-known philanthropist. Last year, he created a $200 million foundation to equip public libraries in North America with computers and the Internet. The plan to give computers to 8,500 of the poorest libraries in the US and Canada was believed to be the brainchild of his wife Melinda and was announced just weeks after a British plan to set up an information technology campus in Cambridge.

Although sceptics denounced the library foundation as a plan to outdo Netscape Communications domination of the Internet market, others saw the projects as an attempt to finally reverse Gates' reputation for keeping his fortune to himself.

Microsoft employs nearly 1,000 people in its Irish operations and Mr Gates has said the company will continue to invest and grow here. "The jobs we account for directly and indirectly in Ireland will continue to go up," he told an international forum earlier this year.