If your taskbar is set to Auto Hide, it may leave a trail of its' border in any window that it overlaps if:
. The taskbar is attached to the side of the screen.
. The taskbar is more than one button high and attached to the top or bottom of the screen.
To work around this problem:
. Right click a blank area of the desktop and choose Properties.
. Select the Plus tab.
. Uncheck Show window contents while dragging.