Your .htaccess in the root of your Zen Cart folder can be used to handle this.


Add something like this, changing the_domain_name appropriately:
Code:
#### insert www if not present
#
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.the_domain_name\.com [NC]
RewriteRule ^(.*)$ http://www.the_domain_name.com/$1 [L,R=301]
#
#### end www fix