When Microsoft rolled out Office 2007, the company made some fundamental changes to its file formats, updating them from binary files to XML-based packages. The default format for Excel 2007 followed ...

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 workbooks, but can also use the XLS format.

Once you make the upgrade to Microsoft Office 2007, both the suite and Windows itself assume you'll hardly ever want to make documents in older Office formats ever again. As most modern office workers ...