Just an update on one of my previous recommendations. I recently got sick of the free McAfee firewall that came with my computer and uninstalled it. This finally gave me a chance to try out the freeware
Comodo Personal Firewall, which uses only a fraction of the resources the McAfee firewall uses. It provides rule based filtering for your network traffic on a per-application basis, giving it a feature set on par with the pay version of Zonealarm.
To my surprise, Comodo also comes with a HIPS component called Defense+. HIPS stands for
Host-Based Intrusion Prevention System. It will basically prevent any unauthorised code from running on your computer or altering any other program or protected file. A HIPS system can effectively prevent infection even from unknown malware. Although it's still a good idea to have an anti-virus scanner to detect and remove any malware that slips through, HIPS software provides a new first line of defence. Comodo comes with a whitelist of safe software which helps cut down on the number of prompts you will receive (which was a huge problem the last time I tried a HIPS system).
Although I've only been using it for a few days, I can already recommend it for anyone looking for a free firewall.