I get asked this in email all the time. I thought it was worth a quick post:
The NAP Server pieces, which are required to do NAP, are included in Windows Server 2008. If you own Server 2008, you own the NAP Server. You need at least ONE Server 2008 computer to do Microsoft NAP.
The NAP Client is included, in-the-box, with Windows Vista. If you own Vista, you own the NAP Client. The NAP Client is shipping on XP within forthcoming XP Service Pack 3 release. This will be "free" to those who already own Windows XP.
NAP the WORLD in 2007,
NAP Release Manager
* Remove the "online" to actually email me.
** This posting is provided "AS IS" with no warranties, and confers no rights.