External minitor problem

Hello,

I'm having this weird problem with my acer aspire laptop, I've been trying to put on duel monitor *Ive done this before and it worked fine* Its all plugged in correctly and I went to my display properties and clicked secondary monitor then i cliked extend my windows desktop onto this monitor and use this divice as primary, but everytime I set it as primary screen will go black on the external minitor and the laptop will become the primary screen again, I was told I could simply right click my desktop and select graphic options and click external minitor but when I right clikc my desktop the "graphic options" dosnt show up as an option anymore (it did when I first bought the laptop) the external minitor works as an extended screen to my desktop, just not a primary screen, Witch is what I want to use it for I dont want to drag stuff from the primary onto the bigger external it just becomes annoying :\ can anyone help me? wat do I need to do to set it as primary?