Textures/Repaints Aircraft.CFG Problem

I am having a problem with the aircraft.cfg file. After moving the new texture file into the appropriate airplane I go into the cfg file to cut and paste and reassign it the next fltsim.xxx number, when I go to save it, it says "Access to......aircraft.cfg was denied"

I am doing it exactly as I did in FS9, so I am not sure if I should be doing it differently in FSX???

Thanks!

Reboot and try again.
Sometimes Windows does that for protection.