File Format Converter for Office 2008
In response to complaints about backward compatibility, Microsoft has created an Open XML File Format Converter that allows Office 2004 programs to view and edit documents created in Office 2008. As more people upgrade to the newest versions, they may still want to share their work with individuals using older version of Office. The XML Converter allows for seamless sending and receiving of information.
Additional troubleshooting help can be found in Microsoft's How to use earlier versions of Excel, PowerPoint, and Word to open and save files from 2007 Office programs