Are these the symptoms of a trojan horse?

When i click on a site, for example yahoo answers that i have searched for in google, i will get taken to a totally different website. For example just before i was taken to a website about locksmiths. A few days ago i did get rid of a virus that was making norton flash up “a recent attempt to Attack your computer has been blocked” and it turned out to be a trojan horse or something. What can i do? norton isn’t detecting anything. Btw i only get taken to a different website sometimes like 20% of the time. And its not certain website i can click on a random website and get taken to a random one. thanks for your help