Need To Add the WWW...how?

I guess that when I installed zen it installed on my site without the www.... so now, the links on the pages automatically don't include that so everything just goes straight to http://sitename.com rather than http://www.sitename.com and I need to change this. I went into the configure files in both admin and the store and changed the defines, but this messed things up and didn't allow me to login to anything. So I'm assuming I need to change something somewhere else....but where?????

OK....well I changed only the entries in the include/configure.php files that were http, not https and it works...but is this going to become a problem in the future?

There are two 'configure.php' files to edit. One is in the store, and the other is in the /admin/ folder.

You'll also need to redirect the old URLs to new: http://www.zen-cart.com/forum/showthread.php?t=151606