Ugo,
Do you have your Samsung set as the 'Primary Monitor' in Windows? Here is an image from this
help setup thread. Just above the bottom red circle you see a checkbox 'Use this device as the primary monitor.' You must select the Samsung monitor, and then you check this checkbox so that games and programs know you want this one monitor to be used.
It may depend on who wrote the program/game as well. If you do not have this checked, some programs may just pick one monitor and it worked for that game without this. But other games/programs don't pick just one monitor, and they ask Windows where you desktop is, and without this checked, Windows says that 'both' monitors are to be used.
Hope this helps..
WBK