Avast Free Antivirus
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Full user review
Avast has been excellent at keeping itself updated on all of the current virus and malware definitions. It's also totally free to update the program itself. It runs totally silent in the background unless something trips one of the alerts.
There are no cons that I've discovered yet.
This product has actually worked better than many of the paid subscription anti-virus programs. I had used Symantec for years until a friend told me about Avast. Now, I'm hooked.