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It seems some virus, spyware or some other malware substituted this Registry Key [HKEY_CLASSES_ROOT\MSCFile\Shell\Open\Command] from a REG_EXPANDABLE_SZ type to a REG_SZ, hence the "Access is denied" because that wouldn't allow to form a proper command.
The solution is to remove the Key and recreate it by importing the following into the Registry:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\MSCFile\Shell\Open\Command]

@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00 ,52,00,6f,00,6f,00,74,00,25,\

00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00 ,32,00,5c,00,6d,00,6d,00,\

63,00,2e,00,65,00,78,00,65,00,20,00,22,00,25,00,31 ,00,22,00,20,00,25,00,2a,\

00,00,00