Thank you very much for your support.
The step that I followed are :
1) Enable the second monitor in
Display Properties --> Settings --> Advanced --> ATI Displays tab
It is a button to switch (for some reason I didn't understand it was to be switched)
2) Change all the DisableDualView in the registry to 0 according the suggestions found in
and restart the computer.
This operation allows to have two displays in
Display Properties --> Settings
and to be able to click on
Extend my windows desktop onto this monitor.