unwanted pop up windows

Good pm to everybody.
When i’m on a site, if i mouve the cursor an unwanted windows open, most of the time advertisement lke:Pc repair, facebook, lazada,etc.
I was told it’s a virus, Avast does not find it! What can i do?
Best regards to all. Rudi

Follow the instructions > https://forum.avast.com/index.php?topic=194892.0