How do I get rid of unwnted pop up websites?

For the last week, every time I go out to a website, an unwanted site pops up and I have to cancel it to go back to the site I had entered. Sometimes, it is almost like it comes from my original choice, but it is not and I have to be very careful. I have had my hard drive cleaned and searched for possible malware but nothing is found and the problem continues. Do I have a setting which needs to be changed?

Mary Lou