The startup flight is seriously bugged in its own right. Either disable the "Fly Now" screen or change the start up flight to something else. I use the Cessna, and select the 777 from the Fly Now screen after FSX loads.
Instead of telling you to RTFM, I'll explain the problem. There is a bug in FSX whereby loading the SAME ENGINE VARIENT causes memory corruption within FSX. It may result in a crash, systems freeze, or if it appears to be working normally, instability later on leading to a crash.
If you launch the saved flight directly from the FSX saved flight folder in My Documents by clicking on the *.FSSAVE file, FSX will load the aircraft directly.