Keep the Taskmanager open, launch FSX and try to not if Couatl.exe launches at least for a brief moment.
Or, try to enable The Simconnect diagnostic mode: open notepad, and copy the following text:
[SimConnect]
level=normal
console=1
RedirectStdOutToConsole=1
OutputDebugString=1
; file=c:\simconnect%03u.log
; file_next_index=0
; file_max_index=9
Save the file as SIMCONNECT.INI in this folder:
Documents And Settings\YOUR LOGIN NAME\Documents\Microsoft Flight Simulator X Files
At the next FSX launch, you should see a text window with diagnostic message, that should tell what's going wrong. Let me know what you are getting, so we can have an idea what's happening.
Only the first few lines are important: if the window scrolls too fast, you can use the "Pause" key on the keyboard to pause the scrolling. Post here only the first 10 lines or so of the diagnostic, but it's important you post the first 10 lines.