Hrmm weird, I'm running FSX with SP2 installed with out any problems, first off mind if I ask some question (I've done tech support before ;) )
1. When was the last time you updated Direct X 9? while it might say 9C or what ever as pointed out by a lot they do update it with out increasing the number on it, so you may need to update DX.
2. When you say nothing has changed you mean no hardware changes right? Is it possible that you need a driver update?
3. What are your system specs?