Can you share with us, what window is it? Is it present in any form on the taskbar. Saltgrass explained an option that should work, but the window has to have "focus". If you open taskmanager can you see the application or process that has created this Window? And as Reghakr said, it shouldn't reappear after a restart unless it's in your startup folder, or located under HKLM or HKCU->Software->Microsoft->Windows->CurrentVersion->Run you can download something like autoruns or even look in system information, expand software environment and select startup programs, this may give you a clue why it keeps popping up when your system starts. Also take a look in Control Panel->All Control Panel Items->Display->Screen Resolution make sure you are not extending your display to a non-existent monitor if so change it.