In a Windows 7 based computer with Office 2003 (Pro) and Microsoft Office Compatibility Pack for newer versions of Word, Excel, and PowerPoint File formats already installed, the following error appeared when the user tried to open Excel workbooks that were created in newer Office 2007 or 2010 versions (*.xlsx) : “The (excel) file is not in a recognizable format…..". Uninstalling and re-installing MS Office compatibility pack didn’t resolve this issue. Finally, after doing some research, the following solution fixed Excel’s “unrecognizable format” error message:
How to fix ‘File is not in a recognizable format’ error message in Excel.
1. Right-click on any “.xlsx” file you want (e.g. ‘Book1.xlsx’) and go to “Open With” > “Choose Default program”.
3. Navigate to "C:\Program Files\Microsoft Office\Office12", to select the “Moc.exe” * file and click “OK”.
* MOC = Microsoft Open XML Converter.
4. Now open your “.xlsx” workbook. Your problem should be gone!