two things you should try, each after backing up the respective files first.
1. Backup and delete C:\Documents and Settings\Username\Application Data\Microsoft\FS9\fs9.cfg
and load again.
2. Open FS9/scenery.cfg in notepad, scroll to the very bottom/end of the file and see if the latest entry looks correct. It should be similar to this, ?? are numbers:
If it doesn't look similar but messed up, you should delete this set of lines , save and see if FS runs. If it does add Vienna by hand as described on the support page.