That doesn't sound good, but maybe something altered your dll.xml file which holds a dll entry for the 707. Without it, it will crash the sim when trying to load the plane.

Go to C:\Users\your name\AppData\Roaming\Microsoft\FSX and open the dll.xml with a text editor.
Look for this entry to be present.

Code (]      <Launch.Addon>
           <Name>Captain Sim 707 Sound</Name>
           <Disabled>False</Disabled>
           <Path>Captain_Sim\707\cs.sound.dll</Path>
     </Launch.Addon>[/code):

If it's there, you can check if it gets loaded correctly when you delete the [Trusted] section in the fsx.cfg. Just make a backup of the file, then open it with a text editor and delete all entries in that section. FSX will then ask for confirmation to load the dll. If he doesn't ask anything, the dll.xml itself is corrupted and we have to build a new one, which isn't too hard, so don't worry. If you don't see that entry, just add it but make sure to keep the [code]</SimBase.Document>

line at the end of the document and don't add anything more than those lines above.
For safety reasons, backup the dll.xml before working on it.

Some addon programs alter this dll.xml file and some of them fail to leave it intact.