Microsoft Knowledge Base Article 300957 contains the following summary:

This step-by-step article describes how to prevent unauthorized users from accessing either a stand-alone computer, a network computer, or a remote computer. Each of these types of computers has different security requirements and the topic of security is complex. This article describes some simple procedures to secure your computer.

For another user to access your computer, they must either be assigned to an administrative group or they must have the appropriate security rights and permissions.