A group policy should have done the trick. You may want to investigate why the group policy did not make the change as your computers may not be receiving other group policy updates as well.
To address your question, there's a range of potential solutions. The easiest way to ensure this occurs is to establish the group policy for user logins. You can either have the registry change load during the windows login procedure or set up a group policy for users to have a default Web site.
I've included a .reg file setting that would set the default start page for Internet Explorer and the location for Firefox. You can load this into a vbs script upon login for all users. This would ensure that even if the user changes the setting, it would get reset at each login.
For Internet Explorer:
gets inserted into the pref.js file in the profile folder.
This was first published in December 2009