I have learned anti-virus programs are a very personal thing.
I almost hate to bring this thread back up to the top since it is slightly off topic. But, I agree, AV programs can be very personal. I have tried lots and lots. I have installed many on other people's machines saying "This will work". I hope they do. But honestly, we go from program to program just hoping it works: Kaspersky, Adaware, Spybot, Malwarebytes, Security Essentials, McAfee, and Symantec.
I used to routinely run Adaware, SpybotS&D, and Avira (free). All updated, My brother used my machine downloading movies and got me infected and hacked. So much for those three? Not so fast. When I was disinfecting another virus laden machine I went through AV after AV trying to clear it. What worked? Security Essentials. Who would have thought.
I got a free offer from my cable provider for Symantec. I loaded it up and went into my VM sandbox, browsing risky places because I thought I was "safe" in my sandbox. Then out of the blue Symantec comes up with "Virtualbox.exe has tried to execute this.malicious.program...". I was impressed. I have never seen a host OS program catch a virtual guest incursion via web browser. I hate Symantec. I bought Norton Utilities (including AV)years ago from them with the 1 yr AV subscription. Once that year was up do you think there was 1 day they didn't try to remind me I needed to resubscribe? I uninstalled the entire thing, vowing to never return. But their new program was much better.
Each one of these AV programs excells at certain things. When I am confronted with an infected machine I pull all of them out and work through them 1 by 1. By far the best practices is block ads, browse safely, keep a backup.
TL:DR, Symantec picked up a threat from a web browser inside a sandboxed virtual machine. Being consciously safe when you browse the web trumps any AV program.