On your machine with both word 2003 and word 2007 installed, the most recently installed version gets the default file type.
You can change the sequence as follows:
To set Word 2003 as the default, open Word 2003. Click on Help and then Detect and Repair.
To set Word 2007 as the default, open Word 2007. Click on Office button -> Word Options -> Resources -> Diagnose.