Setting up Network on Public WIFI network

Hi wondering if anyone can suggest a way to do this.
I am on my building public WIFI network that i have no control over. I dont like the idea of anyone getting into my computers.
I have a linux ubuntu box that i use as a file server, htpc etc. This machine has a ethernet card and a WIFI card. For now the ethernet is connected to my router and the wifi is connected to public wifi. I also have other computers and laptops. I want to use this linux box as my firewall/proxy through which all my other computers connect wirelessly to the internet without losing much speed. If needed i can move from linux to windows 7 too.