if you are using joomla 1.5 & cb 1.2 and want to redirect & to cb 1.2 login module here is what you do:-
1. go to joomla global configuration from your admin account and deactivate allow user registration (set it to "NO")
2. Then using your ftp client locate cb 1.2's login module (i use cb 1.1 and its module login is mod_cblogin.php found in "path_to_your_joomla_1.5_directory/modules/mod_cblogin/)(i don't know if its the same for cb 1.2.. try your luck but the next steps will remain the same)
3.download the file on to your hard drive and rename it to default_login.php *NO EDITION OR HACKS TO THE CODE*
4. Now from your ftp client open the directory of the template you are using for joomla. (here i use the example of joomla 1.5 default template called ruhk_milkway)
5. the path should be "path_to_your_joomla_1.5_directory/tempelates/rhuk_milkyway/html once you are in the html directory of your tempelate create this folder if it is not there "CREATE FOLDER ='com_user'"
6. Open the com_user folder and create another folder named "login"
7. Now open the login folder and just upload the default_login.php that you created in step no. 3 and thats that.
NOW all your unregistered/unlogged users will be forced to cb's login module if they click on links that requires registered users and also from this page they can create a new account through cb registration page.