try this pathch:
in this file
/wp-content/plugins/wpec-related-products/wpec-related-product.php
replace:
function on_wpec_related(){
global $post;
with:
function mopu_on_wpec_related(){
global $post, $wpsc_query_vars, $wpsc_variations, $wpsc_custom_meta;
replace:
while($related_product->have_posts()) : $related_product->the_post();
?>
<li>
<div class="prodbox <?php echo $wp_query->current_post ; ?> product-<?php echo wpsc_the_product_id();?>">
<div class="prodimg"><a href="<?php echo wpsc_the_product_permalink(); ?>"><img src="<?php echo wpsc_the_product_thumbnail(); ?>" alt="" title="" /></a></div>
<div class="prodname"><a href="<?php echo wpsc_the_product_permalink(); ?>"><?php echo wpsc_the_product_title(); ?></a>
<?php if (wpsc_have_variation_groups()===false) { ?><div class="prodprice"><?php echo wpsc_the_product_price(); ?></div><? }?>
</div>
</div>
</li>
<?php
endwhile;
with:
while($related_product->have_posts()) : $related_product->the_post();
$wpsc_custom_meta = new wpsc_custom_meta( get_the_ID() );
$wpsc_variations = new wpsc_variations( get_the_ID() );
?>
<li>
<div class="prodbox <?php echo $wp_query->current_post ; ?> product-<?php echo wpsc_the_product_id();?>">
<div class="prodimg">
<a href="<?php echo wpsc_the_product_permalink(); ?>"><img src="<?php echo wpsc_the_product_thumbnail(); ?>" alt="" title="" /></a>
</div>
<div class="prodname">
<a href="<?php echo wpsc_the_product_permalink(); ?>"><?php echo wpsc_the_product_title();
?></a>
<div class="prodprice">
<?php if(wpsc_product_on_special()) {
?>
<span class="salestrike"> <?php }
echo wpsc_product_normal_price();
?></span>
</div>
<?php if(wpsc_product_on_special()) :
?><div class="prodpricesale"><span class="salered">?<?php echo wpsc_the_product_price(); ?></span></div><?php endif;
?>
</div>
</div>
</li>
<?php
endwhile;
