I have been recently failed by Avast as well.
After reading this thread, I made a switch from Security Essentials, to Avast.
After running Avast for a few months now, and I thought everything as going fine. I started having some redirecting problem in my browser, and got a little suspicious. I check my Avast log history, and for over a month, my scheduled scans have been detecting viruses, but Avast doesn't appear to do anything with them. It didn't even give me any sort of obvious indication that they were still on my computer.
I cleaned them up through Avast, and also ran Trend's House Call to clean up my computer a little more. I also recently installed Malwarebytes to clean up my computer even further.
Unless there is some sort of Avast setting I didn't select to automatically clean and warn of virus detection, I'll probably head back to Security Essentials as that has never let me down.
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Prilosec class action