Avast Free Antivirus
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Full user review
"Simply the best available."
Outstanding protection for $0.00. You have to go out of your way to get whacked when using Avast! Paired with Comodo (free) firewall, complete protection.
I've used them all and none compare. If there was a 6th star, I'd add it.
Although a custom install will clean it up, it installs modules that you may not need, e.g., IM, P2P, etc.