What has taken over my computer?

Suddenly I have lots of programs popping up and interfering with the web pages I'm on from yahoo searches. I also have google toolbar and ask toolbar. After selecting a site from my yahoo search, other sites pop in suddenly announcing that I have won a prize, offering me alternative search results which if you mouse over to x it out pop up anyway and are obviously tricky and hard to get rid of. I have turned pop ups off and have Norton Internet Security and it has not found anything or corrected the problem. What is this and how to get rid of it? Thanks for your help.