Toshiba laptop, see ext. monitor & LCD?

Got a Protoge’ 7200 laptop and we need it for a presentation. We have it hooked to an overhead. The overhead connects to the video port in the back of the laptop. Problem: When we connect, we must reboot and then ONLY the overhead screen works, not the LCD. We need the LCD to work for navigation purposes, AND the overhead. Had someone suggest F5 toggled, but it didn’t work. We need both screens, not just one or the other. Anyone got any suggestions? Running Win98.