Microsoft Security Essentials
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Full user review
"I like it."
Simple installation, configuration, and operation. Seems to be as effective as anything else I've used.
Could use a little more documentation.
I'm generally one of Microsoft's tougher critics, but when they finally stepped up to the plate with an A/V, they did Security Essentials right.
It's easier to install and configure than most, and requires no attention after that, so I like being able to set and forget it on family members' PCs. Formerly a user of Symantec, Avast, and AVG (in that order), Security Essentials is now what I would recommend. Have had it running on three PCs with no problems since shortly after it was introduced.