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 Open an XLS File in Excel. XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save ...

While there is a way for you to open two Excel workbooks at the same time, you can also open multiple spreadsheets in separate windows. Learn how!

There are ways to re-set or establish associations between file and program in Windows 10.