If the ads look like they're meant to be there (as on AB) then either Adblock Plus isn't working properly or, as Aquariel suggests, it's not got a filter list activated.
However if the ads seem to be appearing on pages where you'd not expect them (perhaps, say, on the BBC website) then the problem is undoubtedly related to malware. Download, install and run the free version of Malwarebytes Anti-Malware :
https://www.malwarebytes.org/antimalware/
When installing it, take care to deselect the option to take a free trial of the 'Premium' version. Don't be surprised if a scan takes over half an hour to run. When it reports that it's found malware, click the button to view the results, put a tick against everything listed and click to remove it all. (Hint: If there's a long list of things that need to be ticked, simply right-click on the first one to obtain a 'check all' option).