From C properties: 18,2GB used
From C folder all items (including system and hidden) selected properties:
14,1GB
Where is 4GB?
I remember having 2 or 3 GB free space before System Restore but now have 1GB?

Also after several restores I discovered that one program has stopped
working because its files have been renamed. I have bunch of files with the
same name except (2) or (2)(x) something in the end as a result of renaming.
Is there any way to know which of them is the original file from the time
before the first restore? I assume if I could rename the files to correct
ones then the program would work again since System Restore shouldn't have
deleted anything from there?
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You can't know everything about everything.