According to one of the press releases of Microsoft,the Windows XP SP3 is under development and will be released later this year so really a good news Windows XP users.
Here is what the press release reads-

The Juniper Networks Infranet Controller, the policy management server at the heart of the Juniper UAC solution, will be able to leverage the built-in security assessment capabilities and the SOH protocol provided by the NAP agent built into the Windows Vista and Microsoft Windows XP operating systems. Microsoft Network Policy Server (NPS) brings a rich ecosystem of NAP health agents and will be able to interoperate with the Juniper Networks UAC solution in heterogeneous network environments.

Microsoft NPS can either act as a policy server or be leveraged by the Juniper Infranet Controller for rich endpoint information. Microsoft is announcing that Windows Vista supports this protocol today and Windows Server 2008 and Windows XP Service Pack 3 (SP3) will support it as well later this year.

News source: