Infostealer.Onlinegame

Norton was doing it's quick scan at idle time yesterday and found Infostealer.Onlinegame and successfully removed it. I am looking at the site with the information and it has been around since 2008, so my Norton Product knows about it.

My question is if the Norton Product is "monitoring" my system, how did it get there in the first place ?! This upsets me that it got to my system in the first place. I get all the things about "Norton toolbar, etc. to protect me", yet, an old Trojan was somehow downloaded and installed to my system. How? Why? Shouldn't my Norton product have found and stopped it?

Am not happy about this. I can understand some new never before seen virus, but something from 2008?!

Someone please explain this to me. Is the AV products out there all just "smoke and mirrors" ?