how to ? query_posts for specific tag and ignore private

I have a custom query on my main index page – I’m having 2 problems with this query that I hope someone can help me work around

1) how do I make this ignore any private posts?
2) my 'tag' => 'paid' argument is not working?

thanks,
Mike

$args=array(
    'cat' => 4,
    'tag' => 'paid',
    'orderby' => 'rand',
    'posts_per_page' => -1,
    'showposts' =>15
);

query_posts($args);

while(have_posts()) {
    the_post();