You shouldn't need to run THUG Pro as admin.
With that being said, navigate to \Users\<username>\AppData\Local\THUG Pro
Back up your saves, if you have any, and delete the THUG Pro directory.
Try running the installer again.
If the installer still throws an error.
Run regedit and navigate to HKEY_CURRENT_USER\Software\THUG Pro\Settings
and delete the key named thug2_path
Try running the installer again.