Virus not being found?

I ran a full system scan. It detected no viruses, but I'm am pretty positive there is one. I think this because whenever I go onto google on any web browser (ive tried them all except Opera) and i click a link, it directs me to a different webpage that is completely random. It is normally shopping websites. I then back out and click the link again and it takes me to the link i originally clicked, it works second click usually, sometimes it takes 3 clicks. Anyways, i was wondering why my Norton security did not detect this, and why it still hasnt.