Acer 5000 screen problem

After following all feedback, I still continue to have the problem. My son's laptop is windows xp. His laptop was connected to the external Envision screen. When I hooked up my laptop, after ree booting the Envisoon screen, the screen boots up and says Windows XP. After the message disappears it states NO SIGNAL. I click the F5 key which shows display. Three boxes pop up. I need to check the middle box when it shoes a laptop screen to an external screen. I clikc on it but it doesn't change.