Top leaders

William H Gates

William H Gates dropped out of Harvard in 1975 to concentrate on his new company Microsoft, which he had set up with his childhood friend Paul Allen. Microsoft focused on developing software for a brand new market - personal computers.

Microsoft's first big breakthrough was being approached by IBM to make the operating system for its new PC in 1980. Gates won the right to keep the copyright for what became MS-DOS and then, Windows. The rest is history, with Gates eventually becoming the world's wealthiest man.

Gates, who has been virtually synonymous with Microsoft, has also been a controversial figure and is seen as both brilliant and ruthless. The company's tactics, criticised by some as anti-competitive, have been challenged in courts within and outside the US.

In 2000 Gates stepped down as chief executive, a role taken on by longtime Microsoft executive Steve Ballmer. Many found it hard to imagine Microsoft without Gates at the helm but in practice the transition was smooth. Gates, as chairman and "chief software architect", remained ubiquitous within the company anyway.

A bigger shift happened this year when Gates formally retired to focus energies on his philanthropic projects via the Bill and Melinda Gates Foundation. He remains as chairman, but many are concerned about his less engaged role just as the company faces challenges in a changing market.

Apple chief executive Steve Jobs co-founded the company with friend Steve Wozniak in 1976. By the early 1980s the two had made the cover of Time magazine and become the first of the new wave of young geek entrepreneurs.

Success brought its own storms and tempests within the company and in 1985 Jobs was replaced by former PepsiCo executive John Sculley and then Gil Amelio. Jobs left and formed NeXT Computer and also set up Pixar, a computer animation company, after buying the animation division of LucasFilm. Meanwhile, the post-Jobs era began a long, slow downhill slide at Apple as the company lost market share as well as a clear product vision.

In a surprise move, Apple bought NeXT in 1996, bringing Jobs back into the fold. Apple's - and Jobs' - star began to rise again with the introduction of the iMac, iPod, iTunes and the iPhone.

Despite his insistence that he is well, Jobs' bout of pancreatic cancer in 2004 and thin appearance at a recent conference prompted worried comments from the industry. No current chief executive is so connected to the success and definition of their company as Jobs.

Michael Dell

Michael Dell was picking apart and reassembling computers by the time he was 15 and displaying his fledgling business skills by selling enough newspaper subscriptions to earn a BMW and a PC.

No surprise then that he set up a company called PCs Unlimited in his university dormitory room - building and selling computers. By the time he was 19 the company was so successful that he dropped out of college to focus on the business, soon renamed Dell Computers.

Dell pioneered the online-ordered, custom-built PC as well as the low-inventory, fast-turnaround business model, generating massive profits, redefining the industry and making Dell the most profitable computer company in the world.

By 1992, Dell was the youngest-ever chief executive of a Fortune 500 company.

With a firm manufacturing anchor in Ireland the company has remained one of the county's largest employers.

By 2004 Dell was ready to hand over the reins to Dell's president and chief operating officer Kevin Rollins.

Just under three years later, with the company navigating rough waters, Rollins resigned and Michael Dell was back.

At the time, Dell board director Sam Nunn said: "The board believes that Michael's vision and leadership are critical to building Dell's leadership in the technology industry for the long term."

Carly Fiorina

Carleton S Fiorinabetter know as Carly, is a Stanford graduate in medieval history and philosophy who dropped out of law school to get an MBA and an MIT master of science in management.

Her business skills quickly found a place in the technology industry - she joined telecommunications giant AT&T in 1980 as a management trainee and rose to senior vice president. She ended up at AT&T spin-off Lucent in 1996 running several divisions, and by 1998 was ranked number one in Fortune magazine's most powerful women in business list.

A year later, she was headhunted to run tech behemoth Hewlett-Packard, where she became a very visible, and often controversial, chief executive.

As the industry went into a steep downturn she presided over 7,000 layoffs and took on the task of "reinventing HP".

A key decision on her watch was to buy computer manufacturer and services company Compaq.

After weak performance, the board dismissed Fiorina in 2005, amidst leaks of confidential board memos and fiery board disputes.

Since her departure HP has done very well. Critics say this is due to her departure; supporters credit her groundwork for producing results after she was gone.

She has recently written a book on her HP experiences and has appeared in lists of possible Republican vice-presidential options for John McCain.