I have been using AVG for years and years and as far as I know, I have never had a virus it didn't pick.
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infamous Nortons (which was absolutely rubbish)
Allow me to say something fair about Norton (now Symantec). Compared to the old Norton Antirvirus, I am quite happy with the Symantec Endpoint Protection. To be fair to Norton (Symantec) antivirus, they really is dealing only with virus. And no spyware, adware, malware. In this regards, they lose out to the competitor who does cover a bit of everything.
Few months ago, one of the computer at work was infected with a virus originated from Indonesia, and none of the popular antivirus software could detect it, that including Symantec, AVG and a few more. There is only one (at that time) that detect and kill the virus. The name is not very popular in this forum.
www.norman.com Also at work (yes we have many stupid computer users who just are too good to introduce new viruses to us), I have come across many interesting virus that AVG fail to detect (or sometimes detected, but not cleaned.) Given time and the virus sample, sure AVG can detect and clean it. But at that very moment, it fails.
Virus is a tough business. Big or small, good or bad, I respect each and every one of them. Without them, we can throw away the internet.