I've been using AVG for the last 7 years or so. Have zero complaints aside from the rare false-positive every few months. It's also in my experience a very low footprint, so you don't notice it hogging resources.
I've used adaware on occasion, though since I switched to Firefox + NoScript + AdBlock, the only thing it ever finds is cookies, so I haven't bothered except for once a year or so just to confirm things are still peachy.
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