second display problems

hi
i am having trouble with activating my secondary display. I am using WinXP, graphic card is HD 2400 Pro. monitor is connected with the VGA cable, and TV is connected with an S-video cable. i need to extend (or duplicate) the desktop, but am unable to. I can have only one display active at one time. When i try activating the second display in the display properties/settings (or in catalyst control center) my monitor screen flashes once and the second display is deactivated again, with no error messages. i installed the latest driver for my graphics card, but this did not help. i have used this option with no problems with a much older graphic card (radeon 9250). please help.