You can make Excel always open old spreadsheets in Protected View. Enable or disable Protected View using in-built setting, Group Policy, Registry.

How to Tell If a Worksheet Is Protected in Microsoft Excel 2003. Protecting a worksheet in Excel 2003 prevents others users from editing it. For example, if you have tracked and charted your ...

Microsoft Office 2010 includes a new "Protected View" that, by default, will open all Outlook attachments and documents downloaded from the Internet in "read-only" mode.

If you have sensitive data you'd like to keep from prying eyes in a Microsoft Excel file, the easiest way to protect it is with Excel's built-in password tools.