Win2K comes with an inbuilt Radius server that is free. If you want a AAA server from Cisco specifically, we have ACS that'll run on Windows or Unix, it is not free however, but your account rep will be able to tell you more about that.
I presume that you do have some sort of VPN device to work with. If you want something free then you should be able to add the usernames locally onto whatever the VPN device you're using, which eliminates the need for an external AAA server.
