Faced a weird issue with my new windows 7 laptop today. Whenever I downloaded a docx word document or pptx or xlsx it was getting saved as a zip file. The zip file had a lot of xml files. That reminded me how all the UI elements in windows are stored as xml files. So MS word is just a tool which will take these xml files and create the required document out of it.
But the question for me was how to open this zip file in MS word. The fluke I tried worked out for me that is, just change the extension. E.g. if you downloaded abc.docx, and while download the document got downloaded as abc.zip, you just need to change the name of downloaded file back to abc.docx.