Courtesy of Bioversity International:

"Some of you already ran into files that are suddenly unreadable with MS Office 2003 (or earlier versions):

a dialog box alerts that the download of a Compatibility Pack is required because those files have been generated with the latest version of MS Office.

Be informed that the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats can be downloaded from:

http://www.microsoft.com/downloads/details.aspx?FamilyID=941b3470-3ae9-4aee-8f43-c6bb74cd1466&displaylang=en 

By installing the Compatibility Pack on Microsoft Office 2000, Office XP, or Office 2003, you will be able to open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007"