thumbnail url

wp_get_attachment_thumb_url() returns garbage for me.
It mixes a web url with an absolute url.

http://mydomain.com/images//home/.fielder/myaccount/mydomain.com/images/2008/06/13923_untitled-1_123_468lo-150×150.jpg

How do I fix this? How do I get the thumbnail of one attachment in the post?

My code:

<?php
$files = get_children('post_parent='.get_the_ID().'&post_type=attachment&post_mime_type=image');
  if($files){
					 $keys = array_reverse(array_keys($files));
					 $j=0;
					 $num = $keys[$j];
					 $image=wp_get_attachment_image($num, 'large', false);
					 $imagepieces = explode('"', $image);
					 $imagepath = $imagepieces[1];
					 $thumb=wp_get_attachment_thumb_url($num);
					 //Display image
?>

					<img src="<?php echo $thumb; ?>" width="156" height="103" alt="<?php the_title_attribute(); ?>" />
<?php
}
 else { ?>
					<img src="<?php bloginfo('template_url'); ?>/images/pic_empty.gif" alt="<?php the_title_attribute();?>" width="156" height="103" />

<?php } ?>