Security and WIN 2K VPN

Question: What security (software) can be used to protect a server directly connected to the Internet that will allow me to allow the Windows 2000 Server VPN (Routing and remote access) thru only? I currently use a Sonicwall Pro 300 for security butit will not forward Port 0 (neither will Zone ALarm Pro 3) which is used in a VPN connection (thought this was wierd but verified in Knowledge base.

Also would be nice if it could be applied to only one NIC on a mutlihomed system!

Thanks for any info!