Installing OS X Mountain Lion but not closing applications

I am trying to install mountain lion and it says it's ready to install, however a drop down menu appears which says 'all other applications must be closed before you install OS X.' I click 'close other applications' but the same drop down menu appears 5 seconds later. I have also tried clicking 'cancel' and restart but the menu appears again. Finally I've tried going to normal restart but it says instillation will be cancelled. Can someone advise please?