avast FAQ https://www.avast.com/faq.php
FAQ videos https://www.avast.com/faq.php?q=video

Avast 2015: Using the Virus Chest https://www.avast.com/faq.php?article=AVKB21#artTitle

you did remove any previous used AV before installing avast?
General: Uninstalling a third-party antivirus software https://www.avast.com/faq.php?article=AVKB11#artTitle