Steve Ballmer, the new president of Microsoft and Bill Gates' heir apparent, has the demeanour of a drill sergeant rather than of an executive at the world's leading software company.

A tall, big man with a booming voice and a sharp tongue, Mr Ballmer (42) once said, "to hell with Janet Reno", the attorney general, in connection with the Justice Department's anti-monopolies case against Microsoft. Like Mr Gates, he usually puts his foot in it when he opens his mouth.

While tact may not be Mr Ballmer's strong point, his intense competitiveness and drive have been instrumental in making Microsoft a towering presence in software.

As executive vice-president, he played a key role in building the company's marketing prowess. Microsoft controls more than 90 per cent of the market for personal computer operating systems, and its tactics have earned it the reputation as a corporate bully, an image that will hardly be dispelled with Mr Ballmer's promotion.

"He's a trained killer," said Rick Sherlund, an analyst at Goldman Sachs. "Bill plays hardball and the only guy who plays as hard is Steve Ballmer."

Mr Gates persuaded his friend to join Microsoft in 1980 at an annual salary of $40,000 (£28,600) while Mr Ballmer was at Stanford Business School. Unlike his Harvard-dropout boss, Mr Ballmer stayed on to graduate in applied mathematics.

His 6 per cent stake in Microsoft long ago made him one of the world's richest men, with a current value of $14 billion, although that is nowhere near Mr Gates' $50 billion.

Mr Ballmer is Microsoft's first president since Michael Hallman, a former Boeing and IBM executive, resigned in March 1992, after serving less than two years. Mr Gates then replaced the position with a committee of three executive vice-presidents including Mr Ballmer. Four years later, that was expanded to nine executives, including Mr Gates and Mr Ballmer.

Mr Ballmer will be expected to boost the "discipline and performance" of all Microsoft's businesses, Mr Gates said, as the company makes a push into the corporate market.

Mr Gates said that the change would give him more time to help Microsoft extend its Windows technology to handle the largest computing tasks and to develop software for speech recognition and a range of new hardware devices. In reviewing his calendar for last year, Mr Gates found he had fallen short of his goal of spending at least half his time with Microsoft's products group.