There are a couple of things concerning any security admin regarding its users like don't do anything stupid and keep your software(like OS, antivirus definitions...) up to date.
For the security admin, these things might be under her/his control if the machines are located on the corporate network.
The admin will make sure tha the OS' are patched on a timely basis, install on every user machine a personal antivirus and a personal firewall. He or she will keep the antivirus definitions up to date.
When these machines leave the corporate network for a period of time, and need access to the corporate network from remote locations, the admin is clueless about their state of health.
Were the latest security patches installed ?
Was the antivirus kept up to date, or even worse, is still the antivirus program enabled on these machines ?
Is the personal firewall still enabled on the users' machines ?
Have those machines become a sort of gateways for the user's home network, thus serving as a potential path to the corporate network for an attacker once the user uses a VPN connection to access the corporate network ?
Unfortunetely, there aren't by default any practical means available with ISA for the security admin to check the remote VPN users machines' health status(when I say practical, I exclude the administrative nightmare VPN Quarantine made available with ISA 2004/2006 by Microsoft, solution which will make most of the people abandoning the idea of using a remote access VPN quarantine with ISA).
NAP is a hot topic right now.
If you search for NAP, you may found out that this will be available on the future version of ISA, called TMG. But TMG is far away at this moment.
Luckily for the ISA admins, an enterprise grade VPN quarantine solution for ISA is available for quite a while now. This solution is called VPN-Q 2006. You can reach it at its vendor site, http://www.winfrasoft.com/. It will work with the VPN protocols available on ISA, PPTP and L2TP/IPsec. And very important, VPN-Q 2006 is a very easy to use and to implement solution.