Microsoft Office Compatibility Pack
Files created with Office 2007 cannot be opened with earlier versions of Microsoft Office. Microsoft now offers a free "Compatibility Pack", allowing older versions of Office to open, edit, and save documents, workbooks, and presentations in the new Open XML file formats.
By installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, you will be able to open, edit, and save files using the file formats in newer versions of Word, Excel, and PowerPoint . 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.