It depends on how the ads are delivered to your: while browsing or through the desktop. If they're on the desktop, then I recommend scanning your computer for possible infections. You can use any antivirus from the following list: http://software.informer.com/search/antivirus
Secondly, use the app store from your browser (Extensions or Add-ons) and look for AdBlock Plus.
You can visit the following website and it will detect automatically the version that needs to be installed on your computer: https://adblockplus.org/