(FORTUNE Magazine) – Here it comes: the real trial of the decade. No, not another O.J.-style circus or a made-for-the-tabloids nanny trial, but rather the high-stakes antitrust lawsuit United States v. Microsoft, which, barring a last-minute delay, is set to begin Oct. 15. Unlike those other trials, this is a case whose outcome will likely have widespread ramifications--not only for the software and computer industries but perhaps even for the future of antitrust regulation itself.

The first thing you're likely to discover once the case is up and running is how much antitrust has changed since the last time you paid attention to it. Remember the IBM case of the 1970s and 1980s? That 13-year fiasco is the great, hovering presence in antitrust--the one thing no one ever wants to see again. Take, for instance, the astonishing speed with which the Microsoft case has moved: The Justice Department's antitrust claim against Microsoft was filed less than five months ago. And federal judge Thomas Penfield Jackson seems utterly determined to keep things moving; for instance, he's limited each side to only 12 witnesses. Here's one big surprise so far: Bill Gates--who is himself a great, hovering presence in this case--is not scheduled to be a witness, even though much of the government's evidence consists of E-mails written either by him or to him. (That could change, however, once the trial begins.)

More important, the sprawling nature of the IBM case wound up changing the intellectual foundation of antitrust law. Over the past 15 years or so, the courts have become much more rigorous about what constitutes a violation of the Sherman Antitrust Act. Companies that gain monopolies through legitimate means deserve praise rather than condemnation, the courts now believe. And the courts now hold that even extremely dominant companies are allowed to compete fiercely against rivals without necessarily running afoul of antitrust laws. The key question now is whether a monopolist has a legitimate business reason when it takes an action that hurts a smaller rival--or whether the action's sole purpose is to hurt the rival. This, indeed, is the central issue in the Microsoft trial: Have Microsoft's actions--such as folding its browser into its Windows operating system--been done for business reasons? Or was the sole purpose to damage Netscape? The same action could be either legal or illegal, depending on the motive. That's what makes this case so difficult to parse.

With much of the evidence still under seal, it's hard to say who has the upper hand. The trial judge has seemed to lean in the direction of the government--but then again, the court of appeals made a ruling in June that would suggest it is favorably inclined toward Microsoft's chief argument, namely, that it is well within its rights to include "Internet functionality" in Windows.

What has been most striking in the weeks leading up to the trial is the sheer, unmitigated combativeness with which Microsoft has put forward its arguments, not just in the courtroom--where its open sneering at Netscape is a regular part of the proceedings--but in the court of public opinion. The company will not even concede that it has monopoly power, despite its 90% share of the desktop operating system market. ("If Justice can't accomplish that," laughs San Jose antitrust lawyer Richard Gray, "they all deserve to be fired.") "It's a campaign," shrugged a Microsoft spokesman one day outside the courtroom--a campaign to persuade not just the public but perhaps even the Supreme Court, which is where this case will almost surely wind up.

"If Microsoft wins this case," says William Kovacic of the George Mason University Law School, "you're going to see a major retreat in the Justice Department's willingness to bring future antitrust cases." But what if Microsoft loses? That may be the most difficult question of all: What should the remedy be if the company is found to have attempted to maintain its Windows stronghold by illegal means? (Remedy, not punishment, is the real issue here.) The government has suggested in its briefs that remedies could include forcing Microsoft to include the Netscape browser in Windows and eliminating "exclusivity" provisions in its contracts with Internet service providers. But if that is how this plays out, it will be widely viewed as a victory for Microsoft. "It will be seen as delivering magic beans for the cow," says Kovacic.

Still, short of breaking up the company--something no one in the government is suggesting--what other options are there? Here's the biggest surprise of all: No Microsoft opponent has yet come up with a plausible remedy. "I find it hard to believe you can have a case where a company is found to have violated the law, but there's nothing you can do about it," says Robert Bork, who is serving as a consultant to Netscape. So does Bork have any ideas? "Nobody knows the answer to that--yet."