You will notice the avast chest is in three sections:
1. Infected Files, speaks for itself, any files detected by avast that you chose Move to Chest as the action.
2. System files, back-up copies of important system files. There are (generally) files in the System Folder of the Chest: command.com, kernel 32.dll and wsock32.dll. During the installation, avast! copies some critical system files into the Chest, under the "System files" category. Those files might cause the operating system to crash if they get infected by a virus. If needed, those files can be restored from the Chest to their original location. Should an unknown virus infect the computer despite the extensive protection from the avast! antivirus package and alter an important system file, it can then be easily restored to its original state.
3. User Files, an area for the user to import suspicious files that haven't been detected so they can do no harm and can be sent by email to Alwil for further analysis.
Files are safe into Chest, can't harm your computer.