I found the problem. Windows 8 treats changing instances diferently than Windows 7 and XP. Office 2010 require different file open to create different instances. It's native mode was open in the same instance. Office 2013 forces each workbook into it's own instance. However Windows 8 treats switching instances diferently. Windows 8 ignores the screenupdating = false when it goes between instances. How do I force office 2013 to start a workbook in the current instance?