JP,
Sounds like you're up against Vista's UAC security. This protects files which Vista thinks are 'important', and includes any .cfg files located inside the Program Files folder, including the aircraft.cfg files in FSX.
One way around this is to move the .cfg file to your desktop, edit it and then move it back.
Another method, quoted from Fr. Bill-
http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1241428131In Windows Explorer, highlight your ..\FS9 folder, right-click, and edit the Permissions to allow "Full Access" with "Administrator Rights."
If you ever decide to completely reinstall FS9, do yourself a HUGE favor and do NOT install it to the default \Program Files location!
Instead, install FS9 to its own, unique folder such as:
C:\FS9
or even D:\FS9 if you have a second hard drive or partition...
He refers to FS9 but the same holds true for FSX.
Dave
Always back up any file before editing....
At that time [1909] the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.