Check Windows Device Manager to ensure that the monitor is not showing as disabled. If it is, enable it.
Are you able to set the resolution correctly via Windows Display Properties and then restart your system to check if the same issue occurs?
thank you for your reply. The issue was not the monitor. Few minutes ago I uninstalled the Creative video camera (for Skype) that's working at 640x480, and now screen resolution is stable at 1240x1024. Probably I have to update that driver