Not a lot to go on, but:
AFCAD is probably not finding your fs9.exe where it has been told to look. You need to go into the AFCAD setup again and direct it to the right path.
Did you remember to add the FS9.0 > FS9.1 upgrade after you made a fresh install? I have caused myself much head-banging by forgetting to do that.
I also experienced crashes like yours, when I started using Win7, even though I took great care to install both my FSX and my FS Classic outside the \Program Files\ default location.
These days, FSX is still in C:\FS10\ and my FS Classic now lives on a separate HDD, at G:\FS Classic\. I run the FS Classic in XP3 compatibility mode, and both installs are set to 'Run as Administrator', to stop UAC deciding what I am allowed to do on my own computer.
For years I have been using the free defragger from Auslogics
http://www.auslogics.com/en/software/disk-defrag/
and when I moved from Win XP to Win7 I was horrified by the mess Win7 seems to make of storing files - it looks to be a complete dog's breakfast, after just a 10 minute flight. In Win XP I defragged every week or so, but in Win7 I routinely defrag before starting either sim, and I no longer get that sort of problem.
Usually, when you put the FS9 installation CD in, if it finds a valid installation, it will ask, 'do you want to repair or uninstall?' Did you see that at any stage?
Good luck, hope some of this helps.
MikeW