I’ll give you the simple, direct answer:
I had the exact same problem and fixed it by selecting “Schedule a Bootime Scan” from the Avast main menu.
Then reboot your PC.
This will run a complete system scan before any programs load into memory. Avast will find infected files - either try to fix them and if that doesn’t work, move them to a folder where you can at least go back later and see which files were affected - that way you can decide if they are important files or not.
Good Luck 8)