Can anyone recommend authentication server software? More specifically, one of my clients has a router with a VPN endpoint, but the only authentication is via a key exchanged between the VPN client and router....beyond that, there is no way for someone connecting to authenticate with the windows network.

More details:

Network type: Win2k server with 2 Windows NT and 2 Windows 2000 clients
Router: Linksys RVxxx series

Thanks in advance !