Check this post
https://www-secure.symantec.com/connect/forums/what-symantec-network-access-control-client
Symantec Network Access Control client is:-
The Symantec Network Access Control client evaluates whether a computer is properly protected and compliant before it is allowed to connect to the corporate network.
The client ensures that your computer complies with a security policy that your administrator configures. The security policy checks whether your computer runs the most recent security software, such as antivirus and firewall applications. If your computer does not run the required software, either you or the client must
update the software. If your security software is not up to date, your computer may be blocked from connecting to the network. The client runs periodic checks to verify that your computer continues to comply with the security policy.
It Does:-
It ensures that your computer's security settings conform to network policies. Security settings can include software, software configuration settings, signature files, patches, or other elements.
How it works?
The Symantec Network Access Control client validates and enforces policy compliance for the computers that try to connect to the network. This validation and enforcement process begins before the computer connects to the network and continues throughout the duration of the connection. The Host Integrity Policy is the security policy that serves as the basis for all evaluations and actions.