If all your visitors shall access your website with a URL like www.domain.com and not without www, use the following apache rewrite rule for redirecting them.
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www
RewriteRule (.*) http://www.%{HTTP_HOST}$1 [L,R]
The apache rewrite rule can be added in a .htaccess file in the website root directory or if you use ISPConfig 2 or 3, you can also add the rwrite rule into the apache directives field of the website.
I think there should be a forward slash after {HTTP_HOST} like so:
RewriteRule (.*) http://www.%{HTTP_HOST}/$1 [L,R]
Cheers,
Alan