Hello, in case you're still having problems. You're probably using an AMD graphic card. The cause is your AMD graphic drivers. If you're using the current drivers 13.9 or higher, downgrade it to 13.4. atiumdag.dll is causing the crashes. Hopefully AMD fixes it soon.
Anyways, hope this helps.
I am having this exact same problem. I installed a clean version of Windows 8.1 and FSX Deluxe/Acceleration (Gold Version). In Free Flight, I choose the Boeing 737, and change the Airport to KSFO (any airport, really), and when I click Fly Now, it gives a fatal error. It's driving me nuts. Windows 8.0 didn't have this problem.