Your antivirus is wrong, you shouldn't believe anything that antivirus products says, they use questionable heuristic methods only to advertise being able to catch more virus than the competition.
Either turn off the antivirus, and follow the message that has been posted countless number of times:
- DOWNLOAD AND INSTALL with the antivirus entirely OFF
- Uninstall GSX. Reply YES when asked if you want to remove the Addon Manager and the Couatl Engine. Reply YES to both questions.
- TURN OFF your antivirus. Don't just exit from its interface, but turn off the real time protection. Actual wording changes between products, however, it's the option to scan every file that gets open in real time, without user intervention.
- Reinstall GSX with the Antivirus TURNED OFF. Don't launch FSX yet.
- Configure the antivirus to exclude the following files from scanning:
YourFSXFolder\bglmanx.dll
YourFSXFolder\fsdreamteam\Couatl\couatl.exe
YourFSXFolder it's usually C:\Program Files\Microsoft Games\Microsoft Flight Simulator X, if you installed it in the default folder.
- Now you can turn the antivirus back on, and launch FSX.
- Check you are seeing BOTH the "Addon Manager" menu AND the "Couatl powered products" menu under the "Addons" menu
Or, much better, switch to an antivirus that works and doesn't mistakenly flag legit programs as threats, like the FREE and always updated Microsoft Security Essentials.
What has probably happened, is that your antivirus has been updated, and it now mistakenly detect the executable as a threat.