Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
You can work around this issue by performing a "Reload User Aircraft" once after loading the aircraft in question. This will "unlock" all of the .xml file protections so that you can now load/edit/save .xml files...I know if one tries to open an .xml file that is part of the plane you are currently flying & try to save it will tell you you cannot, because it is using it.
Seeing as you've installed FSX to the default location in Program Files its almost definitely UAC sticking its nose in where its not wanted.
A bit late now unless you fancy re-installing from scratch but the best way to avoid this is to not install FSX in the default location.
Another drive is best but anywhere outside the Program Files folder will do such as C:\FSX or whatever.