Microsoft Flight Simulator X: Steam Edition won’t start, it says I have to be a Windows Administrator. Why do I have to start Microsoft Flight Simulator X: Steam Edition as a Windows Administrator?
Posted by on 08 July 2015 01:35 PM

If you use the User Access Control ("UAC") features of Windows Vista and above, unless you are logged on as an administrator, you may be prompted to permit multiple program changes during installation. This is normal and can be avoided by temporarily disabling UAC via the Control Panel.

Further information about Windows UAC can be found here:

FSX: SE requires you to run it as an Admin in order to access addition folders on your hard drive where add-ons not available on Steam may have been installed.