Calin,
It is probably a pop-up ad trying to get you to buy their anti virus. Since you use Symantic, which is an excellent anit-virus application, you don't need their software. But it may be a ploy to get you to click and then it's possible you could get some malicious download. With Symantic don't let your subscription expire and continue to update the virus definitions frequently if you do not allow the program to do automatic updates.
A good site to get free spyware is the Kim Komando site. You should run at least two spyware applications on a regular basis. I like to use Ad-Aware and Spybot. Both of these and other can be found at kim's web site. There are others there as well. See:
http://www.komando.com/bestshareware.asp#adware
One more thing. I like to use the Mozilla Firefox browser in place of Internet Explorer. Firefox can be downloaded at:
http://download-firefox.org/
Both IE or Firefox have pop up blocker options, which can help with problem you described.