Download a new installer from the Get New Links in the Extended Download Service:

Check-in to Your Profile > Product > Extended Download Service > Get New Links

NOTE:
1. Make sure your AV is OFF/disabled while you download it
2. Make sure your AV is OFF/disabled while you install it.
3. Run FSX using the "Run as administrator" (especially if FSX is installed in the default location - C:\Program Files, or C:\Program File (x86) - in Vista/Win7/Win8/Win8.1).