Whilst everyone is changing from the old 2003 version of Excel to the new 2010 version, some people will still need to provide spreadsheets in the older version. If this is for a single or occasional file, in the Save As dialog box simply change the File Type in the bottom box to Excel 97-2000 Workbook, beneath the File Name.
If you are saving in the old format on a regular basis, it will be simpler to change the default file format. Click on the File tab and select Options, and on the Save page, change the default File Type at the top. When everyone is converted, change this back to Excel Workbook.