Hi'
You have to change it two places.
Subject:
Change in www.yousite.com/language/english/user.php
Find the line Quote:
define('_US_USERKEYFOR','User activation key for %s');
Change to:
Quote:
define('_US_USERKEYFOR','any text you want');
Content
Change in www.yousite.com/language/english/mail_template/register.tpl
That shoud be easy, when You see the file.