You want to look into IPSec. It is an excellent NAP enforcement and provides network level peer authentication and protects communications using the cryptographic security services.
More information on IPSec.
http://technet.microsoft.com/en-us/network/bb531150.aspxStep-by-step guide for NAP IPSec enforcement in a test lab and other NAP resources.
http://www.microsoft.com/windowsserver2008/en/us/nap-technical-resources.aspx