From what I gather it is free, I saw no software limitations or trial bull.
Say you install it on one machine that is always on. Suppose it's 192.168.0.1, after the proxy installation you start it up, than right click the green proxy icon on the taskbar and select configure, for Proxy Binding you enter 192.168.0.1, click everything off except for HTTP or possibly POP3 and you click OK.
Now you go to all client machines, open IE properties, connections, LAN, proxy server, you enter 192.168.0.1 and port 6588.
Now go to your firewall, close port 80 for all except for 192.168.0.1. On pix you must have an access-list limiting the outgoing connections, simply remove port 80 statement and replace it with:
access-list inside permit tcp host 192.168.0.1 any eq 80
and you're done. You'll need one for pop3 as well.