You don't need monopoly power to be guilty under anti-trust law. IIUC, Microsoft is at more risk of being found guilty because it has an existing monopoly that it can leverage.
Sherman Act, Section 2
Every person who shall monopolize, or attempt to monopolize, or combine or conspire with any other person or persons, to monopolize any part of the trade or commerce among the several States, or with foreign nations, shall be deemed guilty of a felony, and, on conviction thereof, shall be punished by fine not exceeding $10,000,000 if a corporation, or, if any other person, $350,000, or by imprisonment not exceeding three years, or by both said punishments, in the discretion of the court.
Don't worry about the pocket-change; the main discourager is injunctive relief, triple damages and legal costs.
ENFORCEMENT OF SECTION 2 OF THE SHERMAN ACT:
THEORY AND PRACTICE (exerpt)
By William F. Adkinson, Jr., Karen L. Grimm, and Christopher N. Bryan
Section 2 also proscribes “attempt
to monopolize.” Attempted monopolization requires proof “(1) that the defendant has engaged in predatory or anticompetitive conduct with (2) a specific intent to monopolize and (3) a dangerous probability of achieving monopoly power.” Although this paper—and, indeed, most of the legal and economic debate—focuses on monopolization, much of the discussion applies to both of these closely-related offenses.
The anticompetitive conduct requirement is assessed using the conduct standards of the monopolization offense, although courts have observed that conduct that is illegal for a monopolist may be legal for an aspiring monopolist, as certain conduct may not have anticompetitive effects unless undertaken by a firm already possessing monopoly power. Moreover, the “specific intent” to monopolize does not encompass “an intent to compete vigorously,” rather, it entails “a specific intent to destroy competition or build monopoly.” Finally, the “dangerous probability” inquiry focuses on the same factors used to assess monopoly power in monopolization claims, although a “dangerous probability” of attaining monopoly power generally can be demonstrated with less market power than is needed for establishing actual monopoly power.