Why not uninstall FSX, and then reinstall it in the location on the C: drive that you already set for it? Doing that won't affect any of the settings you have done for FSX, because uninstalling it won't remove anything from the C:\Users\<YourUserNam>\AppData\Roaming\Microsoft\FSX folder, so the settings will carry over to the install of FSX in the new location. Doing that would remove all the restrictions that Windows is placing on the FSX install in C:\Program Files (x86).