Pull in post with certain tag

Hey,

I have this code:

<?php 

			// Categories of promos to pull in
				$categories = get_post_meta($post->ID, "categories", true);
				if($categories) {
$catID = $categories;
$categories = "cat=".$categories;  }

				// OR use the items below:

				// manually add images instead of using post thumbnail or custom field on other posts
				$add_images = get_post_meta($post->ID, "add_images", true);
				// manually add image URLs, used with above
				$add_urls = get_post_meta($post->ID, "add_urls", true);
				// manually add image titles, used with above
				$add_titles = get_post_meta($post->ID, "add_titles", false);

				// Posts per page
				$posts_per_page = get_post_meta($post->ID, "posts_per_page", true);
				if(!$posts_per_page){ $posts_per_page = "-1"; }
			?>

			<!--// updated 6.20.10 -->
            <?php if (have_posts()) : while (have_posts()) : the_post(); ?>

			<?php endwhile; endif; ?>
            <!--// updated 6.20.10 -->

			<?php

				$count = 1;
				$items = "";

				// if custom field add_images is not set use post query
				if(!$add_images)
				{
					// page size does not really apply since more posts are returned
					// than displayed since a post thumbnail is required
					$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
					query_posts($categories . "orderby=date&order=DES&posts_per_page=".$posts_per_page. "&paged=" . $paged);
					// While there are promos to loop through...
					while (have_posts()) : the_post();
						$thumbnail = get_post_meta($post->ID, "thumbnail", true); // optional custom field inserted on POST being pulled in, not page tempalte
						$thumbnail_link = get_post_meta($post->ID, "thumbnail_link", true); // optional custom field inserted on POST being pulled in, not page tempalte
						if($thumbnail_link){ $url = $thumbnail_link; }else{ $url = get_permalink(); }

						// If the posts have a post thumbnail OR
						// the post has custom fields set for the thumbnail / thumbnail_link
					if(has_post_thumbnail() || $thumbnail)
						{
							if($count % 4 == 0){ $extra_class='end'; }else{ $extra_class = ""; }
							$items .= "<li class='item".$count." project-item ".$extra_class."'>";
							$items .="<a href='".$url."'>";
							if($thumbnail){
								$items .= "<img src='".$thumbnail."' class='post-thumbnail' />";
							}else{
								$items .= get_the_post_thumbnail($post->ID, "gallery-project");
							}
							$items .="</a>";
							$items .="<p><a href='".get_permalink()."'>".get_the_title()."</a></p>";
							$items .= "</li>";
							$count++;
						}
					endwhile;

				}else{ /* custom field add_images is set/used */

					$arr_slider_images = explode("|", $add_images);
					$arr_slider_urls = explode("|", $add_urls);
					$arr_slider_titles = explode("|", $add_titles);
					$count = 1;
					$i = 0;
					foreach($arr_slider_images as $img)
					{
						if($count % 4 == 0){ $extra_class='end'; }else{ $extra_class = ""; }
						$items .= "<li class='item".$i." project-item ".$extra_class."'>";
						if($arr_slider_urls[$i]){ $items .= "<a href='".$arr_slider_urls[$i]."'>"; }
						$items .= "<img src='".$img."' />";
						if($arr_slider_urls[$i]){ $items .= "</a>"; }
						if($arr_slider_titles[$i]){ $items .= "<p>" . $arr_slider_titles[$i] . "</p>"; }
						$items .= "</li>";
						$count++;
						$i++;
					}

				}

				// Only insert the list of items if count > 0, otherwise it will be empty
				if($count > 0)
				{
					$output .= "<div id='container'>";
					$output .= "<ul class='clean clearfix' id='project-thumbnails'>";
					$output .= $items;
					$output .= "</ul>";
					$output .= "</div>";
				}else{
					$output .= "<p class='grid_12'>No gallery items to display.</p>";
				}

				// output promo
				echo $output;

			?>

Which works brilliantly. It pulls in a certain category for a page. You can set the category via Custom Fields.

Is there a way I can alter this so it pulls down certain posts with “tag5” which I have set in Custom Fields?