Windows 7: Windows change out of the blue

I encounter problems with my windows. I have my browser or program windows sized half of my screen. I have a 27" monitor, so don't need full screen windows. But out of the blue they suddenly change to full screen. Also they tend to dissapear half underneath or to the right side of the screen. How is this possible? Does anyone know what happens and to solve this?

I do not really know the cause of your problem. But for the interim, with the cursor on the window, press Win+left (or right) arrow.