Hello and welcome to the Forum
At first a few questions:
Which OS? (32/64 Bit - which SP)
Which avast-version? (Free, Pro, Internet Security)
Any other security related software installed?
Main PC: Win 7 Professional SP1 64 Bit, Firefox 52.0ESR, IE11, AIS 17.2.2288, MBAM 3 Premium (latest version)
Laptop: Win 10 Home Premium 64 Bit, Firefox 52.0ESR, IE11, AIS 17.3.2289Beta, MBAM 3 Premium (latest version)
Avast Mobile Security, Avast Anti-Theft, Avast Cleanup, Avast Backup (always latest version)