This question remains unanswered, relating to your "I later found that some operating system files were missing after the boot."
What were the files and locations that were missing are you sure they were system files or just files in the system folders ?
Before files are restored you should confirm they are clean.
All files sent to the chest (regardless of where they came from) should be in the Infected Files section of the chest.
Are you sure you sent them to the chest 'Move to Chest' and not another option like 'Move/Rename' ?
If you used move/rename they would be in the C:\Program Files\Alwil Software\Avast4\DATA\moved folder.
Can you give some examples of these other files and locations ?
I can't stress enough that before you restore anything there has to be a good reason for it, you believe the files are not infected, etc. and you confirm that.