.docx files showup as .zip when downloading from apache server

Strange problem. Our users are uploading .docx and .dotx files to our apache
server for students to view. When you click the link (either embedded in a
..html file or the direct link from the Apache server's index) Windows XP
displays an 'Open/Save/Cancel' window that wants to save the file as .zip.
The same thing happens if you right-click and choose 'Save Target As.' I can
say that this is not the case when viewing and downloading the files using
Firefox. Word 2007 opens and displays the .docx or .dotx files correctly
without any of this .zip nonsense. Any thoughts?