If you haven't upgraded to Office 2007 yet and have started to receive Office 2007 documents in the mail, then you will probably want to download the "Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats".

This will allow you top Open, edit, and save documents, workbooks, and presentations in the file formats new to Microsoft Office Word, Excel, and PowerPoint 2007.

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