My MacBook Pro has a virus! Help!

Yesterday, I noticed that on most sites, a few words are blue text and are underlined. When I put my mouse over those words, random ads show up. I haven't clicked on any of the ads, so that wouldn't be the problem. I have run an antivirus, and it found 15 infected files. I deleted all the files, and I'm running the antivirus again. It has already found 3 more infected files. The antivirus is Avast!. Please help!!