If you use Microsoft Office XP and 2003 programs Word, Excel, or PowerPoint, Microsoft suggest that you install all High-Priority updates from Microsoft Update before downloading their Compatibility Pack.
Installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, lets you open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007.
You can use the Compatibility Pack in conjunction with the Microsoft Office Word Viewer 2003, Excel Viewer 2003, and PowerPoint Viewer 2003 to view files saved in these new formats.
Download from the MS site here