Did Microsoft Engage in Anticompetitive Exclusionary Behavior?

The court in United States v. Microsoft Corporation paints a vivid picture of Microsoft as a monopolist taking aggressive competitive actions to protect its dominant Windows operating system against a challenge by Netscape. Even if one accepts the court’s factual description of Microsoft’s market position and conduct, a difficult antitrust economics question posed by Microsoft’s behavior remains to be answered. What limits should be placed on the ability of a dominant firm, such as Microsoft, to compete aggressively in defending its market position?