Server 2008 and Vista working better together but until then here is some information for you
Implementing Network Access Protection in Windows Server 2008
This Snack describes the benefits of implementing Network Access Protection (NAP) in Windows Server 2008. It describes how to configure network and health policies for various types of clients. It identifies the options for configuring Dynamic Host Configuration Protocol (DHCP) enforcement for NAP and demonstrates the steps for testing a NAP implementation with DHCP enforcement.