Some times and for strange reasons, the operating system reports that there is no (or limited) access to the operating disk (C:), resulting that neither the programs installed can be run. Before restore your OS, try doing the following trick.
Press Win + X buttons and in the menu choose "Command Prompt (Admin)".
Now type the following commands, one at a time (takes a little time for each). When you're done, try running a program that was not running before.
icacls c: /grant Administrators:(OI)(CI)(IO)(F) /T /C
icacls c: /grant SYSTEM:(OI)(CI)(IO)(F) /T /C
icacls c: /grant Users:(OI)(CI)(GR,GE) /T /C