Can anyone help restore my laptop to full screen display, please?

I have a new laptop which clearly has a mind of its own when it comes to switching between full screen display and three quarter screen display (with black bands to either side and some of the information on the screen being out of sight). Or -- more likely -- its my clumsy fingers pressing something I shouldn't. It's a Toshiba Tecra and it is fitted with Windows 7. If anyone can rescue me in my pathetic plight, I shall be most grateful!