Hiya,
This is what the 'Read Me' file included with the Flight Simulator 9.1 Update says:
Issue with various module .dlls and the Update setup --
Some third-party providers might have updated various files located in the “Modules” folder (for example, C:\Program Files\Microsoft Games\Flight Simulator 9\Modules) of your Flight Simulator installation directory. In certain cases, these updated .dll files will cause the Update to terminate before completing installation. A message will be displayed: “The original file (gps_export.dll) is either missing or has been changed from its original state. The update cannot continue and will now exit”. You need to re-install the any .dll file that came with the original Flight Simulator 2004 setup files before attempting the update installation.
To install the default gps_export.dll file:
1. Insert the “Flight Simulator 2004: A Century of Flight” CD-ROM number 2 in your CD-ROM drive.
2. Click the Windows Start button and choose “My Computer.”
3. Highlight and right-click the icon for the CD-ROM drive containing the number 2 CD-ROM. From the right-click menu, choose Open.
4. Select MSGAME3.CAB and open it using Windows Explorer.
5. Copy the affected .dll (for example gps_export.dll to the Modules subdirectory of your Flight Simulator installation directory. (For example, C:\Program Files\Microsoft Games\Flight Simulator 9\Modules.)
6. Remove the CD-ROM from the drive and restart the Update setup.
NOTE: Previously installed the 3[SUP]rd[/SUP] party add-on software for Flight Simulator may not work. If any action (such as re-installing the 3[SUP]rd[/SUP] party add-on software) causes an overwrite of the updated module .dll file, you will lose significant fixes included in the Flight Simulator 2004: A Century of Flight (update).
Good luck.
Cheers,
Maarten