Perhaps the half-screen is being experienced when you grab the Windows Explorer window (or other) by the Title Bar and shift it to the side of the screen. When that's done, it will suddenly change size and take up exactly half of the screen. This is by design. If you have two windows open and you drag the second window to the other side of the screen, this window will change size and take up the other half of the screen. The function comes in handy sometimes. Writing in one window whilst being able to refer to another is one, As said, drag and drop could be another.
To return a window to its previous size, either click the 'Maximize/Restore Down' button (next to the 'X' button) twice, or grab the window (click and hold down the left mouse button) by the Title Bar and shake the Window about a bit. Magic!
The rules are slightly variable maybe but after a bit of experimentation with the above, you'll soon get the hang of it.
