meulie,
yes it's simple in the $DOCROOT/config/registry.php your horde settings should look like this:
$this->applications['horde'] = array(
'fileroot' => dirname(__FILE__) . '/',
'webroot' => '',
'initial_page' => 'login.php',
'icon' => 'graphics/home.gif',
'name' => _("Horde"),
'allow ...