Joel Klein takes no prisoners. The head of the US government's monopoly-busting anti-trust division fights hard and fights to win. But he is no bully either - he fights clean and he likes to pick on opponents who are bigger than him. And as Bill Gates is now discovering to his chagrin, he has a nasty habit of winning.

When the verdict was handed down against Microsoft on Monday he spoke not like a civil servant who has done a commendable job, but with the zeal of an activist who has just won a campaign. "This landmark opinion - and in the history of anti-trust, this is indeed a landmark opinion - will set the ground rules for enforcement in the information age," he said. "It demonstrates once again that no company, no matter how big or how powerful, can refuse to play by the rules."

The battle against Microsoft's monopoly has been portrayed as a straight showdown between government and commerce. It is a depiction that suits both sides well. Microsoft gets to play the aggrieved business struggling against an anti-entrepreneurial state. Meanwhile the government gets to look as though it is striking a populist blow against bad practice and monopoly.

But it was in the end a fight between one softly spoken, bald-headed man and big business. The 53-year-old son of a Brooklyn postman has been here before. Klein is the most successful entrepreneur in the US federal bureaucracy - the pen pusher who turned the justice department's anti-trust unit into an astonishing profit centre for the treasury by collecting more than $750m in fines over two years, mostly from European monopolists. This is the man who blocked the merger of two of the biggest names in the US defence industry, charged a group of the world's largest banks with running an illegal conspiracy to control the international credit card industry and then picked a fight with Microsoft within the space of six months. Now he has felled the wealthiest man in the world.

Klein may not have the sex appeal of Erin Brockovich. But before the year is out the film rights to the legal war he waged against Microsoft will almost certainly be on its way to the big screen.

His contribution was applauded by his boss, Janet Reno, the US attorney general, who described him as "the man who deserves so much of the credit" for bringing Microsoft to its knees. But the praise comes not just for completing the case successfully but for starting it in the first place.

George Carey, a senior anti-trust lawyer in Washington, said yesterday: "Joel deserves a lot of the credit because he had the courage to actually pursue the case."

Klein, who appeared during important parts of the 77-day court hearing against Microsoft, acted as a sort of field general in the campaign. David Boies, the chief prosecutor with a penchant for red meat and cheap suits, was his chief gladiator. Klein limited his appearances to a few post-victory salutes wearing his trademark Cheshire Cat grin and navy blue suits.

But unlike Boies, Klein made his name fighting for the impoverished, the mentally ill and people with learning difficulties. Growing up in public housing in the New York district of Queens, Klein was an unlikely candidate for the Washington in-crowd. When he was appointed chief trust-buster he was treated with the cold suspicion usually reserved for an outsider, because he was thought to be soft on big business.

Some tried to block his appointment. "The only thing Justice would use its enforcement authority on is the merger of Jupiter with Neptune," said Lloyd Constantine, a former anti-trust official. In 1997 Klein was decried as a "weak" choice by the New York Times.

Now that Judge Thomas Jackson has ruled against Microsoft, Klein's critics are concerned about his strength. They worry that he is overzealous and even an enemy of commerce. When asked whether Klein was still regarded as pro-business, Carey, who once worked with him, laughed. "I don't think there's anyone that thinks that any more," he said.

But Klein rejects any claim that his battle against the commercial big guns is driven by ideology. He said he would not kowtow to the public demands for blood at the same time as refusing to bow down to big business. "The public notion of tackling giants, that's a mistake," he says. "You have to look at the particular facts of a case. I'm not here to tackle big corporations or get in the way of economic efficiencies.

"I don't have this overarching theoretical model I'm trying to lower on the economy," he says. "I'm trying to decide each case on its merits, one at a time, not on ideological grounds." He credits mainstream economic thinkers rather than legal or political ones as his great influences. "My thinking is mainstream economic thinking," he says.

However, Klein does have a healthy mistrust of monopolies. At a gathering of computer industry leaders last October, a sensitive time in the case, Klein said simply: "Monopolies have to be held to a different standard because they have monopoly power."

While his motivations may be eclectic, the vigour with which he wages his battles is all his own. His friends, who describe him as both "scrappy" and "sophisticated", believe that his fighting methods highlight his defining characteristics: his immense intelligence and street-smart aggression. When talks broke down with Microsoft, the company's lawyers blamed the politicking local attorney generals rather than the justice department. Yet Klein is on the record as saying that he would rather go to trial than quietly negotiate financial penalties.

Klein started his career by winning a scholarship to Columbia University, where he earned a degree in economics, and then went to Harvard University law school. He was awarded magna cum laude at both. After working with people with disabilities he went on to become a corporate lawyer before breaking off to start his own company. Like many Democrats in the Clinton era, his career took a new turn when he met the president. Klein and his wife, Patsy Davis, were regulars at the "renaissance weekends" hosted by the Clintons in the 80s. In 1993, he was asked to help shepherd a Clinton nomination through the supreme court.

In the two years he worked at the White House he managed to gain a cameo role in the two greatest scandals of the Clinton presidency. During the Whitewater affair he argued that all White House documents implicated in the matter should be made public. It was this viewpoint that would earn him the wrath of the First Lady. And he was the man responsible for moving Linda Tripp, the woman who secretly taped Monica Lewinsky, from the White House, thus provoking her ire.

The fact that he would become even slightly embroiled in both scandals tells us something about the desire of the man to be where the action is. The fact that he survived them both tells us something about his ability to outsmart his detractors.

But his proximity to power, and hunger for it, has not left him without endearing human qualities. Lloyd Cutler, a friend and former colleague in the White House counsel's office, has said: "He's just about the only lawyer my wife likes. He can communicate feelings and read other people's feelings."

He went through a divorce at the start of the Microsoft proceedings almost two years ago. But his teenage daughter still lives with him when she is not away at school and he often says he wants to go home to spend "quantity time" with her when he leaves the office .

His interests include tennis and reading biographies of famous jurists. The government safely proved that Microsoft was such a beast by simply pointing out that its Windows software is used on more than 90% of the world's computers. Robert Pitofsky, the federal trade commission chairman and an old friend of Klein, says: "He doesn't fit comfortably into any one camp in anti-trust. He takes his cases one at a time."

But the case Klein chose to pursue, against the company credited with making more millionaires than any other in the history of corporate America, was also likely to get more coverage around the world than almost any other. Although Klein has allowed several huge mergers in the telecommunications industry, he is credited with helping the Clinton administration bring anti-trust issues to the front of the political agenda. Some campaigners believe Klein was simply in the right place at the right time. Ralph Nader, consumer activist and some time Green Party presidential contender, says: "He's picked a great time to be anti-trust chief because the level of merger activity is spectacular and that department has reached rock bottom in the past few years."

Monopoly behaviour had largely been ignored during the Reagan years, and is likely to be so again if George W Bush becomes president later this year. Bush has already indicated his disapproval of the Microsoft suit. Klein has said little about what he will do after the November election, whether the Democrat presidential candidate Al Gore is successful or not.

Some friends say Klein still harbours ambitions to be solicitor general, where his supreme court skills would come to the fore. Other analysts argue that he will be only too glad to give up his relatively tiny government salary, estimated at about $148,000 (£90,000), for "10 times as much" as a corporate lawyer. He would also be keenly sought by corporations.

But whatever this new "enemy of big business" decides to do next, he will always be remembered as the man who put Bill Gates in his place.

The state v Gates: Microsoft under siege

1990: US federal trade commission begins investigating Microsoft's "tie-in" sales of applications and operating systems.

1993: Justice department begins anti-trust inquiry into whether Windows has a monopoly in the PC market.

1994: Microsoft agrees to stop punishing PC makers that use rival software.

1995: Microsoft launches Windows 95, which features a free web browser, Internet Explorer. A US court order requires Microsoft to lift anti-competitive licensing terms that require PC makers to install Explorer.

1996: Explorer begins to erode Netscape Navigator's market share, prompting allegations by Netscape that Microsoft is using unfair and anti-competitive practices.

1997: Microsoft is sued for allegedly violating 1994 agreement by forcing manufacturers to promote Explorer. Judge Jackson issues preliminary injunction to stop bundling of Explorer with Windows. Overruled on appeal.

1998: Department of justice and 20 US states sue for breach of anti-trust law. Trial begins on October 19. In November, America Online buys Netscape for $10bn (£6.3bn).

1999: Jackson's findings are sharply critical of Microsoft. He appoints an independent mediator.

April 1 2000: Settlement talks fail.

April 3: Jackson delivers verdict that Microsoft is guilty of violating anti-trust law by attempting to monopolise the internet browser market. Ruling on a penalty is expected in several months' time. Microsoft chairman Bill Gates vows to appeal.