In Excel 2007, use "Save As" and select "Excel 97-2003 Workbook."
The "but..." part is that any new features introduced in Excel 2007,
such as the additional rows and columns in 2007, will not be carried
over to the saved spreadsheet.
An alternative approach is to download and install the Microsoft Office
Compatibility Pack, which should allow you to open the Office 2007 file
format in Office 2003.