go to click here and get the latest drivers for your graphics card.
remove the old ones first via add remove programs.ati control panel first(dont reboot) then the driver and reboot.then install the new driver,reboot and then install the control panel.also try this click here
you dont say which operating system you have but have a look in:
control panel/display/settings/ you should see two screens 1 & 2,click on 2 and then tick the extend my windows desktop to this monitor box.
also for radeon cards have a look in:
control panel/display /settings/advanced/displays and set up your second monitor here.