Force drupal 7 to place "www" in front of urls there is a problem when a use logs in after typing "domain.com", clicks on a internal link that leads to "www.domain.com/somthing" and they are no longer logged in until they navigate to "domain.com" again. I just want the system to force the url to add the www at the beginning.
Adding this to your .htaccess file will do it...

RewriteCond %{HTTP_HOST} !^www\.YOURDOMAIN\.com [NC]
RewriteCond %{HTTP_HOST} !^$
RewriteRule (.*) http://www.YOUDOMAIN.com/$1 [R=301,L]