Windows 2003 Server Pack 1 has a new capability that you might want to look into called Quarantine VPN.

With this technique you can validate that all clients that connect to your VPN meet specific requirements before they actually get access to network resources.  Microsoft has been doing this on their network for quite a while now and they have finally given everyone else that uses their products the same capability.

For details on how to implement it and a more in depth overview on Quarantine VPN read this Technet article.