I'm running Windows 8 by the way.
Nowhere we say that Windows 8 is a supported OS. As you can can see from the System Requirements page of GSX, the only OS officially supported are XP+Sp3, Vista and Windows 7. If you run on an unsupported OS, you are on your own.
This doesn't mean our products will "surely" not run on Windows 8, in fact, many users use them, it's just that we can't support them officially, because they are untested.
Some user reported that, under Windows 8, in addition to your antivirus, you must turn off or properly configure Windows Defender too. Perhaps you are running two antivirus at the same time.
And, your report is not very clear: does the Couatl menu disappear after you start FSX or after a while, or it's just not there right from the start ?
Have you been ASKED permission to run it after you Uninstalled/Reinstalled and excluded the antivirus ?
If you haven't even been asked permission to run it, there are only two possible explanations:
1) You haven't followed the Uninstall procedure to the letter. Specifically, you have skipped the part that directs you to reply YES to the question "Do you want to remove the Couatl scripting engine"
OR
2) Your antivirus is still interfering.