You may have damaged your Word installation. I have my default document save format set to Word 97-2003 Word Document (*.doc), and I don't even have the registry setting you mentioned above.
Try it again, slowly. You don't change the setting when you're saving a file. You click the round Office Button at the upper left, then DO NOT click Save.
Click the "Word Options" button at the lower right of the pop-up menu next to the Exit button. Now click the Save button on the right, and change the default save format. This should change it permanently.
If that doesn't work, you may have to repair your Word installation. Hope this helps.