Windows 7: Why won't my documents etc use the whole screen

I don't know if this is in the right section so i'm sorry if it isn't. I have recently received a Dell XPS 17 laptop and when I open an internet browser or a document/folder and maximise it I can still see about 1cm of my desktop. I don't know how to explain what is it but the close button (X) and minimise buttons etc arent at the top of the screen, they are 1cm below because the browser etc doesnt fill the screen. When I am playing games it uses the whole screen I was just wondering why it is leaving a gap, it used to fill the whole screen then suddenly started doing this. Could anyone help? :S I am also using Windows 7 Home Premium 64 bit and I am using the correct resolution which is 1600x900