I've tried a fresh reboot to install, inheritance polices etc. This fix doesn't work for me. FSX SE is not running in the background, I didn't do it fast .
That would be only one of the possible reasons of the error 5. The other might just be some other program has locked access from the installation folder, most usually an antivirus, and in some cases if might even messed up the file permissions on your drive for that folder.
First, try to reinstall after restarting Windows.
If that doesn't fix it, even if you are SURE the antivirus it's not running now and you are SURE FSX is not running, it might still have caused a mess in your file permissions, denying you from accessing a folder, while the antivirus was still active.
Yes, it seems strange, but I witnessed this first-hand on an user system during a remote assistance session. There are some free utilities you can try to unlock that folder:http://www.iobit.com/en/iobit-unlocker.phphttp://www.freefileunlocker.com/
And tell to the utility to unlock the FSX\fsdreamteam folder, then REMOVE IT manually, and then REINSTALL GSX.