Windows Boards

Good question.  I think the most popular solution for freeware firewall for Windows is avast! 4 Home Edition.  Yes, it is an anti-virus suite, but is also a very decent lightweight firewall.  avast! Home Edition is currently free of charge for HOME users for NON-COMMERCIAL use.

The latest version of this program is 4.7.
Features overview
Antivirus kernel    
Automatic updates
Simple User Interface    
Virus Chest
Resident protection    
System integration
P2P and IM Shields    
Integrated Virus Cleaner
Network Shield    
Support for 64-bit Windows
Web Shield    
Internationalization

Platforms:  Windows 95/98/Me/NT/2000/XP/Vista

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Also there is PC Tools Firewall Plus Free Edition, a powerful personal free firewall for Microsoft Windows that protects your computer from hackers and intruders.

PC Tools Firewall Plus feature highlights

    * Protects your PC as you are working, surfing and playing.
    * Intelligent automatic protection without all the questions.
    * Easy to use. Designed for both, novice and expert users.
    * Advanced rules to protect your PC against common attacks.
    * Best of all it