avast! Free Antivirus is a popular free antivirus for Windows that has just been updated to version 6.

This free antiviral program provides all that you need to protect your computer, including a tool for scanning your hard drive and a real-time shield (the AVG Resident Shield) to prevent infections

 

The free version of this personal firewall prevents hackers and virus to take control of your PC using the security flaws of Windows.

Avast antivirus is a totally free software (for a non-commercial use) and offers all that is necessary to parry virus, worms and trojans. It offers antivirus scan, firewall, intrusions detection, P2P protection... In order to use this software, you'll only have to register freely online with a valid email address.