editing the aircraft.cfg file to add new livery

I don't know why it happens, but I found a way around it.
Steps:
1. Make edits in the .cfg file (in your case aircraft.cfg)
2. Save to a place like your desktop documents just any folder under drive:\users\user. Where drive is put the drive you have installed Vista on (in your case and in general C:\) and where user is put your Windows username.
3. Copy your file to the drive:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes\b747_400 folder. Where drive is put your drive that you installed FSX on (in your case and in general C:\)
4. Windows Vista will ask permission to copy to the drive:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes\b747_400 folder (if you have UAC on) and tell it to continue.
5. It will notice that there is already an aircraft.cfg file there so it will ask you to choose what to do. I am sot sure exactly wht is called, but it is something like Move and replace. It is the first option.

You should then have the new aircraft.cfg file in your folder.

Hope this helps -Orion

Don't feel bad, the same thing happened to me. I have Vista Premium, the OS from hell. I had to copy the .cfg to my desktop, make the changes, move the original .cfg file to a holding folder I created (in case the change scerwed up the program), then move the new .cfg file to the Airplanes folder ( respective aiplane). Once you do that, you can then make more changes within the FSX file. Strange, but it happens.