FSX never did work on my PC

I never could get any aircraft to work properly on FSX. My computer is high grade with a great video card and much RAM. I stayed with FS 2004 for that reason. When I upgraded to Microsoft 7 my FS 2004 changed for the worse. I did all the updates for FS2004. Most of the aircraft I load from fly away do not work or they work but many functions are missing. What's with FSX that only a few of the aircraft that came with it actually work?? I'm pretty frustrated with it. I am a pilot but I'm not a computer guy. Is that my problem??