Redirection from http:// to www.

Since I use the network function, my main blog is only accessible by http://www.domain.tld and the http://domain.tld produces an error message: “The registration has been disabled.” What did I do wrong?
Before i activated the Multiblog funktion it was function like i wanted it, but now the domain without the www. (http://www.domain.tld) is redirected to http://www.domain.tld/wp-signup.php?new=domain.tld

But I would like that the http://domain.tld redirects to http://www.domain.tld
Is the problem maybe in the files i had to change?

Changes Files:

.htaccess
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule . index.php [L]

wp-config.php

define( 'MULTISITE', true );
define( 'SUBDOMAIN_INSTALL', true );
$base = '/';
define( 'DOMAIN_CURRENT_SITE', 'www.example.com' );
define( 'PATH_CURRENT_SITE', '/' );
define( 'SITE_ID_CURRENT_SITE', 1 );
define( 'BLOG_ID_CURRENT_SITE', 1 );

Maybe somebody is able to help me out with this.