Microsoft Knowledge Base Article 815141 contains the following summary:

By default, Internet Explorer Enhanced Security Configuration is enabled on Windows Server 2003. When you start Microsoft Internet Explorer, you receive the following message:

Microsoft Internet Explorer's Enhanced Security Configuration is currently enabled on your server. This enhanced level of security reduces the risk of attack from Web-based content that is not secure, but it may also prevent Web sites from displaying correctly and restrict access to network resources.

Internet Explorer Enhanced Security Configuration established a configuration for your server and for Microsoft Internet Explorer that decreases the exposure of your server to potential attacks that can occur through Web content and application scripts. As a result, some Web sites may not display or perform as expected.

This article describes the effects of Internet Explorer Enhanced Security Configuration in Windows Server 2003.