Welcome, Lorenzo.  :) Sorry to hear about those problems. Can you do me a favor and look into the aircraft.cfg of your 707? Find it at SimObjects\Airplanes\CS_B707-300. Open it with a text editor and post some entries like [fltsim.0] and rising numbers.
The line where it says sim=csx701 is very important, so lets just check what is missing there.

Additionally, can you confirm running the version 1.3 of the 707 for FSX? The installer name is csx701_1300.exe.
Does your FSX have at least SP2 installed?