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.

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 ...

QUESTION:A while back, I installed a shareware program called 602 pc suite 2000. The program must have altered the way I log into Windows Explorer to read my Microsoft Excel files. When I do so, I ...

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.

Likewise, the venerable .xls Excel spreadsheet format gave way to .xlsx, and PowerPoint .ppt files became .pptx files.

How to import Microsoft Excel spreadsheets into Apple Numbers This procedure works with .xlsx and .xls file formats. You can also import .csv and tab-delimited files.