FSX Won't Run! HELP!

Ok, so I just installed Abacus Fighter Pilot 2. Problem is, now FSX won't run. I get the error "The Requested file is corrupt or unable to load. Flight Simulator will now try and load the default flight." OK, I get that. I let it reload. Then I get "Flight Simulator cannot find the default flight file. Please restore the file or reinstall the Flight Simulator." OK! PLEASE tell me where said file is in the Microsoft FS file, and PLEASE tell me why it is vital to FSX running. Or E-mail the file back with instructions to me at starloric@yahoo.com . Thank You!