VPN with NT 4.0 and ADSL Modem

I have a small office network. The office has a windows NT 4.0 server, a NT workstation, and two windows98 workstations.Our office is connected to the Internet using an external 384 ADSL modem attached to one of the 98workstion running analogX as a proxyserver. HOW CAN I SETUP A VPN ON THE NT SERVER. The server does not have a public IP.