Until today, I had no idea that there were command-line switches (options) that you can use when starting Word.
I have both Office 2003 and Office 2007 installed on my PC. All of the doc files were opening automatically in Word 2003, but I wanted them to open in Word 2007 by default. I tried changing the file association, but it wouldn't take.
My final solution was to run winword.exe /r from the Office12 folder. This re-registered the registry keys for Word 2007.
If you are curious about other Word switches check them out here: http://support.microsoft.com/kb/210565