If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

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

You don't necessarily need Microsoft Excel to work with an .xlsx file Reviewed by Ryan Perian This article explains what XLSX files are, how to open them, and which programs are able to convert ...

Go to the File Types tab, scroll down to Microsoft Excel Worksheet (.xls not .csv) and click Advanced. Uncheck 'Browse in same window' (or 'Open Web documents' in place) and OK.