sharing cable modem without sharing file

Just got a cable modem installed in my office which is right next to two other businesses. We want to share the cost of the broadband by sharing the internet. There are 4 computers in one business, two in another and two in the third. How can we setthis up to share the broadband but not share our files on our separate networks? All the businesses are on networks and need to share files between themselves, but we do not want to share anything other then the broadband between each business. All computers are running windows 98 or greater. The cat 5 cable is already installed connecting all three business. Thank You