Download the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats from http://www.microsoft.com/downloads/details.aspx?FamilyID=941b3470-3ae9-4aee-8f43-c6bb74cd1466&displaylang=en
That's all you need.
It will allow you to open such new documents/spreadsheets/presentations in your old Office program. Then, of course, you can save it in the old format.
Does anyone know how I can convert *.docx (word 2007) to *.doc (word 2000)? Is there a conversion tool I can download from somewhere?