Network Access Protection(NAP) is an exciting technology feature of Windows Vista client and the corresponding server which is still known by it's codename of "longhorn". NAP provides the automated mechanism to assess the policy compliance (health status) of client machines before allowing them access to your corporate network resources.
Windows Server 2003 Service Pack 1 provides support for Network Quarantine which is a pre-cursor to NAP. Network Quarantine provides the means to assess the policy compliance of machines which access your network via Routing and Remote Access (RRAS). I recommend looking at Network Quarantine as it is available now and is a supported feature of the operating system. I have blogged about quarantine several times - I suggest reading this article and this article to learn much more about this important technology.
NAP goes much further than Quarantine as it is expected to provide enforcement agents for users who access your network locally - Quarantine is just for remote access.
Details of how to get access to the NAP beta software are available here.
Full details of NAP are available here.