Open FSX .cfg and see if you have duplicate entries for the [DISPLAY] section. If you have, delete ALL sections - yes all, including the lines under the entry, like so -
[DISPLAY.Device.xxxxxx xxxxxxx xxxx.0]
Mode=1440x900x32
Anisotropic=1
Then reboot the sim, see if it loads to the initial GUI . If it does, adjust resolution, filtering and AA preferences to suit, and be on your way.
EVERY time you change drivers you should use the method - FSX has a habit of creating duplicate entries on some systems.
If that doesn't work try deleting the FSX cfg and let FSX build you a new one.