Vista notepad not saving altered .CFG files

Hi all!
I've been trying to install new paints for my default B737. This involves adding lines to the planes .CFG file and saving. But when trying to do so I get this message:
"Cannot create the C:\Program files\......B737_800\aircraft.cfg file
Make sure that the path and file name are correct."
Then I click on the "ok" button and it opens up a windows explorer page showing the other tuxture files for the plane.
So, my question is, how can I continue to save?
I've been trying to install new paints for my default B737. This involves adding lines to the planes .CFG file and saving. But when trying to do so I get this message:
"Cannot create the C:\Program files\......B737_800\aircraft.cfg file
Make sure that the path and file name are correct."
Then I click on the "ok" button and it opens up a windows explorer page showing the other tuxture files for the plane.
So, my question is, how can I continue to save?