Connect to external display

I have a samsung hl-s5687w DLP TV, and a sony vaio laptop with vista home
premium. i am trying to connect the laptop to the tv with a vga cable. when
i connect the cable to the laptop, the laptop display goes blank and the tv
says mode not supported. the first time i connected the calbe i selected the
option to display same image on both screens. my question is how can i
modify the options, so that the laptop display stays on while the vga cable
is connected? another question, is the hl-s5687w compatible with vista?

thanks in advance

AP