1) get rid of Avast, it is incredibly invasive junk ware. It hogs your computers resources, makes it prone to crashing, and break into your secure communications (and botches the break-in) living your computer LESS secure.
It scans all you files, looking for patterns that match antique Windows Viruses that are no threat to you, even if these patterns appear in pictures and files where they could not possible become active.
It provides NO protection from you current (presumed) infection, which was probably caused by "Trojan Horse" attack that fooled you into authorizing an installation.
2) Use MalwareBytes, which is Mac-specific instead of generic, and use it to scan and remove ONCE.