The quarantine (Chest) is in the "chest" subfolder of the Avast data folder (C:\ProgramData\AVAST Software\Avast).
However, the files are renamed and their content is scrambled, so I don't know if it's of much use for you.
Well, in the meantime I found that place myself. If this files there are not original: how can I test them at Jotti or virustotal?
You can't upload from the virus chest - so you have to
Extract (not Restore) from the chest to a location outside of the chest. The reason not to Restore is that this sends a copy back to the original location, if it was truly infected it could well be active (if a registry entry or other means) of running it were present.
You can't do this with the file securely in the chest, you need to Open the chest and right click on the file and select '
Extract' it to a temporary (not original) location first, see below.
Create a folder called
Suspect in the
C:\ drive. Now exclude that folder in the
File System Shield, Settings, Exclusions, Add, type (or copy and paste)
C:\Suspect\* That will stop the File System Shield scanning any file you put in that folder.
Now you can Extract it (a copy) to that location and upload it to virustotal, etc.
EDIT added attached image.