Winform with Splash Screen

I have a real strange problem with a winform application I am writing. I set up a splash screen using this example as a model: http://ift.tt/1BInxVI. Most of the time everything work fine but every once in a while the splash screen closes but leaves a white box on the screen where the splash screen was located. I have to reboot to get rid of the box. Does anyone have any ideas? This is a Windows 7 Pro PC.