I'm pretty sure that if you right-click on said .doc file, you get the option in the menu of "Open with" and the option to browse for the application to open the file with, then chances are (if you are using XP), a window will pop up asking you if you want to associate .doc files with this application.
I've used Open Office for ages and I think that is what I did when I first started using it.
Hope this is of some help.
