A firewall offers important security protection, but only if its configured properly and used appropriately. Any decent firewall should offer basic NAT protection. Beyond that, mileage may vary. For example, a permission-based firewall that gives carte blanche to Internet connecting applications will not be as effective as a firewall that seeks permission each time. On the other hand, inexperienced users who don't know how to respond to firewall alerts will likely need to sacrifice a bit of security in exchange for ease of use. Following is a list of the best free firewall apps for Windows.
ZoneAlarm Free Firewall offers two-way firewall protection combined with customizable application level access permissions. ZoneAlarm Free is purely a firewall so it won't conflict with other security software such as antivirus, behavioral analyzers, or host intrusion prevention (HIPs). The configuration options are suitable for advanced users seeking an a la carte solution, while still simple enough for inexperienced users as well.
ZoneAlarm Free Firewall supports Vista SP1 (32-bit only), Windows XP SP2, SP3 (Home or Professional Edition). Works with most types of TCP/IP connections including Ethernet LAN, wireless LAN, DSL, cable modem, and dial-up connections.
Comodo firewall offers a wide range of features - so much so that it may overwhelm casual users and cause conflicts with other installed security software. However, for those users interested in monitoring all aspects of the Internet connection, Comodo Firewall may be a good bet. The free Comodo Firewall download includes antivirus, but the protection offered by the antivirus program is mediocre at best. Fortunately, you can elect during the installation to simply install the standalone Comodo Firewall. A scaled down host intrusion prevention system (HIPs) is also included. For those that like HIPs the protection won't be quite robust enough. For those not familiar with HIPs, the alerts generated may be a bit much.
Agnitum Outpost Firewall Free provides a basic two-way firewall with standard packet and application filtering. Supports Windows XP and Vista. 64-bit editions of XP and Vista are also supported, but a separate download is required.
Sunbelt Personal Firewall supports Windows XP and Vista 32-bit versions only, though a 64-bit version is on their product roadmap. The free version of Sunbelt Personal Firewall offers only incoming packet filtering - to get bidirectional protection you'll need to upgrade to the Pro version.