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I agree that Avast! and AVG are among the best choices. I have used both, and while I still have AVG on my personal machine, I think Avast! may be the slightly better. AVG is a bit of a resource hog, and I've had a couple false positives recently. It's still good enough that I haven't switched from AVG to Avast!, but if I was picking one at the moment, I would probably choose Avast!.
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