I think I only ever had a fatal error with a clean FSX installed (without any add-ons at all),seemed to be because my computer wasn't powerful enough for it (Core 2 Duo E6600, 4GB RAM and a 320MB 8800 GTS).
Since upgrading to my current computer system (see my specs in my signature), in August last year, the only thing that causes a fatal error in FSX, is changing between my payware add-on aircraft. Although I always switch to the default C172 before switching to a different developers aircraft, after four or five times I get a fatal error (CTD). Nothing else I do causes a fatal error.

I guess we have to consider that FSX was designed for a slightly different technology to what w have today, so that can cause problems, especially when there are no longer any updates or service packs. Just imagine, if ACES Studios was still improving FSX, we might be at SP10 by now. :D

Mark