Excel 2007 tip #1: Make your data visible to other Excel users by saving in Excel 97-2003 format. By David Carter
Files produced by Excel 2007 are in the new .xlsx format. No other version of Excel can read them unless a special compatibility pack has been installed.
If you plan to share your Excel 2007 files with other Excel users, set Excel 2007 to save your files automatically in Excel 97-2003 format. This will allow other Excel users to read them.
How to set Excel 2007 to automatically save files in Excel 97-2003 format:
Start up Excel 2007.