I repeat Microsoft's solution:
For backward compatibility and collaboration with earlier versions of
Microsoft Office Excel, you can use one of two ways to open Microsoft Office
Excel 2007 workbooks in an earlier version of Excel. You can either use the
earlier binary file format (.xls) or the new XML-based file format (.xlsx) to
exchange workbooks between different versions of Excel.
To ensure that a workbook that you save in Office Excel 2007 can be opened
in an earlier version of Excel, you can save a copy that is fully compatible
with Excel 97-2003 (.xls) in Excel 2007.
Users who use an earlier version of Excel can also download the Microsoft
Office Compatibility Pack for 2007 Office Word, Excel and PowerPoint File
Formats to install updates and converters for the earlier version of Excel.
This allows them to open, edit, and save an Excel 2007 workbook in the
earlier version of Excel, without having to save it to that version's file
format first or without having to upgrade the earlier version of Excel to
Excel 2007.
See:
http://office.microsoft.com/en-us/excel/HA100141071033.aspx