I am assuming this is Windows 7.....
If you see a single square in the 2nd position top right of the window, that means the state of the window is Windowed
if you see a pair of squares in the 2nd position top right, that means the state of the windows if full screen.
If yours shows the single square in that position, then my suggestion will apply.
That behavior usually means that someone has expanded the Windowed window to the margins of the physical screen. Grab a corner and make it smaller. It can be tricky because you have to get the mouse pointer in exactly the right position when it turns to a double headed arrow, and while holding it there, move the mouse away from the corner