Sorry for bumping an old thread but I found a pretty good free firewall a few months ago (and yes, it's the full version. Not some freeware firewall that contains half of the features found in the full version). It's
Comodo Firewall Pro. Doesn't take up much as much resources as Zone Alarm firewall or even Kerio's firewall. It only takes up less than 30MB of RAM for me. Pretty effective.
