sharing internet connection with MS Prox

I would like to configure an NT machine with MS Proxy 2.0 and share it with Windows and MAC clients. Since one of the user uses a laptop for home and work which is a pain the uer to reconfigure the browser and/or winsock everythin he comes to the office. What is needed so that the laptop user can do everthing with the Internet connection such as FTP and telnet to the outside world? How about the MAC? Is there a share/freeware that can do a quick NAT for NT without going through Proxy 2.0?