Let's say you want Word 2003 to launch instead of Word 2007. That we can do. This is done with File Associations and plenty of web pages about it.
What you need to know is where you installed Word 2003. Since this is your choice I can't guess that but find your winword.exe and now change the .doc association to that .exe.
I have both office 2003 and office 2007 on my computer , but every time I open a document office 2007 started , I want office 2003 to open them not office 2007 .