A security issue

I recently logged onto my Wells Gargo bank account site. While on the site a pop-up appeared on my screen that said that Wells Fargo was asking me to complete a short survey on my banking experience and the bank was givng a free.gift. The questions were what the bank would have asked--how often I went to the banking center, would I recommend Wells Fargo bank, etc. Since that incident, the same pop-up and appeared on other sites that I logged onto. It looks very convincing that the company whose site you are on is asking for feedback. A free.gift is offered for completing the survey. It is a scam. How can it e detedted and stopped? I am sure my computer is now infected.
Evelyn in Houston