Thanks for the advice. I did as you suggested, and whilst simpropace.dll was reinstalled into the root directory, when I attempted to start FSX I got the same error message returned. Against my better judgement, but driven by utter frustration, I uninstalled FSX then reinstalled it along with SP1 and SP2, and have started the long haul rebuilding all the add-ons that I had accumulated on my system. Whilst I can now start FSX, (hooray) another problem has cropped up