Saving as XML

(File > Save As) select "XML Spreadsheet (*.xml)"
SS of this option is all versions

If you save a workbook in XML you can open and edit the file in XML and still reopen it later in Excel provided you havn't broken the schema.
In Excel 2003 it saves a lot of unnecesarry elements

simple example of saving down the text "saving to xml" in cell "B2".

Excel 2003 does not save the following items
1) Macros
2) Charts
3) Shapes
4) OLE Objects

Excel qualifies the names of the XML nodes it creates with the namespaces from the Microsoft Office schemas
Different nodes in an XML spreadsheet use different default namespaces.