I would do a Boot Scan with Avast. And would also do a second opinion scan with Malwarebytes or SuperAntiSpyware, the free versions…One or the other, You can get them from Download dot com, and set Avast “File System Shields” Actions to: Put In Chest …If you still have problems, Post here again with more info about which Avast you are using and more info on which OS you are using, and which other Security progams you have running alongside Avast.
TC and safe surfing!!