Another questions: do you run FSInn/FSCopilot? If so, you have to check your dll.xml file and put the FSCopilot string at the end.

Do you have the 73x from that known developer? If so, probably your dll.xml file is a mess.

If none of those, so the bad news.

My second step:

1. Uninstall all add-ons
2. Delete all remaining files
3. Uninstall SP2/SP1/FSX
4. Delete all remaining files
5. Assure simconnect.dll files are deleted from Windows/WinSXS folder
6. Disable antivirus and firewall
7. Reboot
8. Install FSX. Run for a few minutes
9. Reboot
10. Install SP1. Run for a few minutes
11. Reboot
12. Install SP2. Run for a few minutes
13. Reboot
14. Install CS757
15. Run FSX

Remember that all these steps should be done with antivirus disabled. Be aware that some a/v, although disabled, run on background. So, if you are not sure, it would be better to uninstall antivirus.

Regards

Schmid