Thanks for your answer.
I can't do that, if I launch the game not-in-windowed-mode, the screen is black (+ out of range info) and I can't cancel it, I can't do ctrl+ alt+del or ctrl+escape, I'm forced to shut down the computer with on/off button, so it doesn't save the CFG.
---------- Post added at 12:21 AM ---------- Previous post was Yesterday at 11:58 PM ----------
Problem solved : I launch the game > black screen + out of range > I do ctrl+alt+del (the screen stays black...) > I do 4 times down arrow (to select the gestionnaire des tâches - I don't know english name for that, "task menu" or something like that) > I close Arma 2.
Then the CFG has now a line at its end : "refresh=75;"
I change it to "refresh=60;" and it works.
Ce jeu est fini à la pisse, as we say in France.