The recently released Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats is an addon for previous versions of Microsoft Office (back to 2000).  From the Microsoft website:

"By installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, you will be able open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007. The Compatibility Pack can also be used in conjunction with the Microsoft Office Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003 to view files saved in these new formats. For more information about the Compatibility Pack, see Knowledge Base article 923505."

The compatibility pack can be downloaded from here