Admin is just that and is not God. That is, even an Admin account must obey permissions so if a file is marked read only or is not owned by Admin then such does happen.
For now there is no God account in Windows so this area will continue to confound many.
The problem now is that the changed permissions mean you might have damaged this install to the point you won't be able to easily recover it back to before the changes. DID YOU TRY SYSTEM RESTORE?
Sorry but the other "I'm the only user" is again something written by many. Again, even the only user must obey permissions.
Just bought Toshiba L75D-A7283, upgraded to 8.1 then got message admin auth needed. I am only user and had admin credentials, so I went into Drive C: properties, security and attempted to change admin priviledges and now Drive C: is access denied and most of my apps are the same. I ordered recovery USB stick but it says fail when I try to us USB option to recovery. Also unable to download programs or access Windows trouble shooter.