For some unknown reason Microsoft decided to make Office 2007 documents, such as .Docx uncompatible with Office 2003 without a patch. Microsoft could easily have made it so you do not need a patch but alas it is Microsoft.

In order to view .DOCx and other Microsoft Office 2007 files in Office 2003 you must download the patch for Office 2003 from Microsoft. Just download it and run it and you should be fine. Also, you can prevent Microsoft Office 2007 from saving your documents in 2007 format by clicking the windows button at the top left of the screen in Office 2007. Go to Word Options near the bottom of the new window. Click the Save tab at the left and Change “Save Files in This Format” to just .Doc.