Broken screen fn f4 wont work

Hello,
I have abroken laptop screen. Machine is HP and running on Vista. I'm trying to connect to an external monitor. If I use the fn f4 route it doesn't work. I can't click through the proces on the screen to change monitors because I can't see anything on it. If I start in safe mode I have no problem, the fn f4 works and I can toggle on and off the external monitor. It doesn't make sense that it works in safe mode and not normal mode. Can't do what I need to in Safe mode...it doesn't give me the options. I've tried changing resolution but this makes no difference to the problem.
Help!