when connect to external projector, its only show either 1 display

anyone can help? im having problem to activate 2 display to be shown when im
doing presentation to external projector. supposingly, we are able to see 2
display on my notebook and the projector.
if compare to windows xp, there would not have any problem. is there any
special instruction in vista so tat we can use 2 side of display when we
connect to projector.
Thank you.