Error 0xD8 means
"The version of %1 is not compatible with the version you're running. Check your computer's system information to see whether you need a x86 (32-bit) or x64 (64-bit) version of the program, and then contact the software publisher."
Which is exactly the same message you would get by manually launching, which should never happen in the first place, and it really doesn't happen.
Are you maybe running FSX in the "XP Compatibility mode" or any of the other "Compatibility" modes ? If yes, turn them them all immediately, they are very well known to cause any sort of problems, and it's not how FSX should be run under Vista/Windows7, and it's not how it's installed by default.
Right click on the FSX.EXE file, and turn off everything in the "Compatibility" tab. And, be sure you haven't done the same for Couatl.exe