desktop viewing - I can't see the bottom part of pages

hi, when i load a program, e.g. garage band (not safari that works) it doesn't allow me to see the bottom of the screen - like the page is too big or it's been stretched, this means i can't get access to some of the major controllers. I've went through the system preferences and I can't work out how to put it back to what it was, i.e back to being able to see the whole screen or at least having a scroller. any help with this would be greatly appreciated.