CTD after restoring FS9.exe.

After upgrading to FS9.1, I had renamed the file FS9.exe and installed the no-CD file with the same name. Reading that this file may not exploit all the enhancements of the FS9.1 patch, tonight I renamed the original one back to fs9.exe and loaded the DVD drice with CD#4. I started OK, but crashed to desktop after initially loading the sim. I've tried renaming fs9.cfg so that new one will be re-built, but no change. Now back with the no-CD version and all's OK.Does anyone know what's happening here? I''d like to reinstall the correct .exe file if possible.Thanks, Bruce.