"Flight Simulator was unable to load some program files and will now exit. Please reinstall flight simulator to restore or repair the program installation."
I recieved this same error after running the program for several days without making ANY changes to my system. FSX crashed during one of the missions, and then developed the error.
This is the solution that worked for me. ...at least for now.
On the first DVD, under "\\program files\Microsoft Games\Flight Simulator X\" are 72 files. On your hard drive under "\\Program Files\Microsoft Games\Microsoft Flight Simulator X\", are 74 (or 75) files (and 25 directories.) Overwrite the 74 files of your hard drive with the 72 files from the DVD.
That's it.