This article discusses the possible legal ramifications of Microsoft’s recent entry into the market for security software, such as personal firewall, anti-virus, and anti-spyware software. Microsoft has encountered antitrust problems in the past by bundling software with the Windows operating system, and some commentators have suggested that Microsoft’s next antitrust battle will be over security software. The purpose of this article is to determine whether Microsoft’s new security offerings could constitute illegal tying arrangements. The article provides a technical background, including a description of operating systems and security software in general, an overview of Microsoft’s current and proposed security offerings, and reasons why security software is needed. It then discusses the hypothetical outcome of a tying claim against Microsoft under Section 1 of the Sherman Act by analyzing Microsoft’s product offerings under the traditional per se tying rule and under the rule of reason. It concludes by presenting several possible remedies if a tying violation were to be found.