I just helped a user to install Office 2007 without uninstalling Office 2003, because we want the ability to use applications from both versions. The problem is after installing Office 2007, the Word 2007 is now the default program to open .doc documents and this not what the user wants.
I tried to reset Word 2003 to be the default application by going to Windows Explorer -> Tools -> Folder Options -> File Types. This is not working.
This one article I found on Yahoo Answers works flawlessly.
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.