If you are using Vista or Windows 7, make sure you install it using the "Run as administrator" option (Right click and select it from the list that pops up).

Do the above, and select the "repair" option when given the choice to repair or remove and do a repair of the installation. Hopefully it works.

If it doesn't work, you can download the installation file again (your current installer might be corrupted) and then do the steps above. To get a new installer: Check-in to Your Profile > Product > Extended Download Service > Get New Links

If the last one fails, you are going to need to do a full uninstall/reinstall as per KB #685, where you need to remove the CS767 entry in the windows registry. Check-in to Your Profile > Product > Customer Support > Search KB or Submit New Ticket. Do a search for 685 (search field C).

ok at last !

now its working, but I had to reinstall FSX in Administrator mode, and THEN the 767 in Ad. mode too.
Expect an hour to install FSX in Ad. mode.