Since the problem is specifically with *.docx files, did you remember to use the Open With command with a Word file in that format?
You should also try repairing Office from Control Panel, which should restore file associations.
Yes, as you can see in the images above the file that properly shows text is a "Word 97-2003" document and the one that does not properly display is a "Word" document. I made sure to use a "Word" document when changing the file association and also went
to my default programs in the control panel to check there. Changing that does not affect the display of docx files.
I have also tried to repair and reinstall multiple times. It has not changed the settings. For whatever reason, Office does not completely uninstall from my computer. This means that whatever is causing the problem may never be removed during installation.
Could it be some registry file left over from old Office versions? I don't know, but it would be nice if Office actually completely removed itself when I uninstall it. Why would Microsoft do such a thing.
I should also mention that using a different user account solved an issue I had with not being able to view help files, but it did not solve the problem with docx files in the preview pane.