Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats (Beta 2)
Exchanging Files Between Previous Releases of Microsoft Office and the 2007 Microsoft Office Release
Microsoft has added new file formats to the 2007 Microsoft Office system to reduce file sizes, improve the recovery of corrupt or damaged files, and improve integration with external sources. To help ensure that you can exchange documents between Office releases, Microsoft has developed a Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats. The Compatibility Pack (Beta 2) is available for Microsoft Office XP and Office 2003.
http://www.microsoft.com/office/preview/beta/converter.mspx
