Welcome to the Avast forum asker have you tried this
"Exclusions" Page
avast! makes it possible to exclude some areas, or even single files, from testing; it means that avast! will not search for viruses there. It may be useful in several cases:
Avoiding false alarms. If avast! reports a virus infection in a file and you are sure that it is a false alarm, you can exclude the file from testing and avoid further false alarms this way. However, it would be nice of you to send us such a file so that we could fix the problem.
Speeding up the processing. If you have a directory on your hard disk that contains images only, for example, you can exclude it from testing by adding it to the exclusions list, and thus reduce the time spent on scanning the files.
It is important to keep in mind that these exclusions affect all tasks, except for the resident protection. If you want to set the exclusion for a single task only, you have to edit the particular task.
Settings
Add. Adds an empty item to the list where you can write the folder or file to be excluded. If you want to select a folder including all its subfolders, it is necessary to append "\*", e.g. "C:\Windows\*".
Remove. Removes selected folder or file from the exclusions list.
Browse. An Explorer-like window will open; here you can select the desired folder or file, without having to type the whole path
Hope this helps