Unlike the Clean Access Agent, the Cisco NAC Web Agent is not a "persistent" entity, thus it only exists on the client machine long enough to accommodate a single user session. Instead of downloading and installing an Agent application, once the user opens a browser window, logs in to the NAC Appliance web login page, and chooses to launch the temporal Cisco NAC Web Agent, an ActiveX control or Java applet (you specify the preferred method using the Web Client (ActiveX/Applet) option in the Administration > User Pages > Login Page configuration page) initiates a self-extracting Agent Stub installer on the client machine to install Agent files in a client's temporary directory, perform posture assessment/scan the system to ensure security compliance, and report compliance status back to the NAC Appliance system. During this period, the user is granted access only to the Temporary Role and if the client machine is not compliant for one or more reasons, the user is informed of the issues preventing network access and may do one of the following as mentioned in the below URL:

http://www.cisco.com/en/US/docs/security/nac/appliance/configuration_guide/45/cam/m_cca.html#wp1130212