Microsoft Knowledge Base Article 888651 contains the following summary:

Microsoft Internet Security and Acceleration (ISA) Server 2004 provides support for the Network Access Quarantine Control feature of Microsoft Windows Server 2003. Additionally, it simplifies the management of this feature by integrating Network Access Quarantine Control with the ISA Server 2004 firewall policy. Therefore, you can manage Network Access Quarantine Control by using ISA Server 2004 access rules.

For additional information about the Network Access Quarantine Control feature of Windows Server 2003, visit the following Microsoft Web site:

http://www.microsoft.com/windowsserver2003/techinfo/overview/quarantine.mspx