Microsoft on Tuesday released its latest foray into security software as a limited beta. Microsoft Security Essentials, known in development as Morro, is limited to 75,000 downloads in four countries: the United States, Israel, Brazil, and China.

Security Essentials contains all the basic features that users have come to expect from free security software: multiple built-in and customizable scan options, a scheduler, automatic definition file updates, a real-time defense shield, and rootkit...