Any files in the chest should be deleted from 'within' the chest process and not from windows explores as I believe this is what you are trying to do. First the chest is a protected area and would stop that, add to that since avast 4.8 there is also a self-defence module to stop avast files, etc. from being deleted/modified, etc.
So you need to Open the Chest first and then the Infected Files section of the chest and select the files you want to delete (nothing that has been there for less than three weeks) and then delete them, you shouldn't get any problem deleting from within the chest.
