setting up vpn server

I am setting up a vpn server which is part of a 3 computer network in a small office. The client wishes to access the server from home using win xp home edition, while each office computer is running xp professional. the only unit which should have internet access is the vpn server but the other units need to share a limited number of files. how can i ensure that no one is able to connect to the dsl internet via the other computers.