If your Excel files are opening in Notepad on Windows PC, here's how you can reset the settings to make .xls, .xlsx, etc files open in Excel.

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.

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.