Sounds like spyware, aka malware. Not strictly virus.
First off, only use 1 AV Scanner. You shouldn't have two such programs running as this will impact performance and probably cause conflict between the two. I'd go for AVG over Symantec every day.
Anyways, back to the malware. Grab Windows Defender from the Microsoft Downloads section. It's a free malware detection\removal program that runs realtime and is quite good. It has other features that can help you with your issue.
If that doensn't shift it, grab these apps;
<a class="postlink" href="http://www.safer-networking.org/en/mirrors/index.html" onclick="window.open(this.href);return false;">http://www.safer-networking.org/en/mirrors/index.html</a>
<a class="postlink" href="http://www.bleepingcomputer.com/combofix/how-to-use-combofix" onclick="window.open(this.href);return false;">http://www.bleepingcomputer.com/combofi ... e-combofix</a>
Also, if you're using Internet Explorer click on Tools, Manage Add ons , then look in there for anything that suspect
let me know if you still can't shift it.