Avast! Home Edition is one of the best free anti-virus applications. This edition is free for non-commercial & home use only. A serious defect is how Avast lets you configure exclusions, which is not very easy.
Yet, even more serious is that Avast seems unable to resist some virus attacks against itself. I have more than once found my PC occupied by a virus that stopped and even deleted the Avast executable, leaving me without protection!
No matter how many the advantages of Avast, situations like these made me switch to AVG Anti-Virus Free.
Avast Features: Anti-spyware built-in, Web Shield, Anti-rootkit built-in, Automatic updates, Strong self-protection, Virus Chest, Antivirus kernel, System integration, Simple User Interface, Integrated Virus Cleaner, Resident protection, Support for 64-bit Windows, P2P and IM Shields, Internationalization, Network Shield.