So I just re-installed Windows and FSX on my PC and now when I try and make and save any changes to a plane's cfg file, it says access is denied.

 

I tried right clicking on the FSX exe file within the main folder but that didn't make any difference.

 

I'm fairly sure this is a simple fix, I just can't recall what it is....

That's probably because of UAC. What you can do is save it into another location like My Documents then copying it back to it's original location and click yes if user account control asks you if you want to allow the change.