Proxy on a switch

My boss would like to deploy proxy servers to all 10 of our locations, and I’m in charge of configuring and deploying them. Most locations just have a simple Belkin switch that I can plug the ubuntu box into. The other 3 computers all run Windows XP. I want to run all http traffic through the ubuntu box. My boss only want employees to access our certain sites. I am having trouble doing so. Currently I have a Ubuntu 10.10 box. I installed squid and Web content control. The setting are all at the defaults. How do I go about running all http traffic through this ubuntu box so that the other 3 computers only have access to these sites? Thanks for your time.