Avast Free Antivirus
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Full user review
"Very Effective Antivirus"
Hasn't let anything slip thru yet.
Warns you when you're getting ready to visit a website that has been reported as having malicious software on it.
Easy to configure
A little reading is required when first setting up, because it is so configurable.
I used to have AVG, and it was also a very good, free antivirus, but it wasn't very configurable. I like to have the ability to easily configure my antivirus for the situation I'm currently in. AVAST let's me do that. I highly recommend.