Hi DJ,
I assume you are using XP SP3 if you are using NAP on XP computers. Perhaps you installed the beta version of NAP for XP SP2 which is no longer supported. If so, I'm not sure if the Group Policy setting described below will work, but it might.
The enable quarantine checks setting can be turned on using Group Policy. You can find it under Computer Configuration\Policies\Windows Settings\Security Settings\Wired Network (IEEE 802.3) Policies. Just create a new "Windows Vista" policy and this will also be applied to computers running XP with SP3.
You'll find it on the Security tab, PEAP properties.
I hope this helps,
-Greg