add_action('pre_get_posts','WPECfix', 10); // hook in just before wpsc_generate_product_query will run
function WPECfix(){
        if ( $priority = has_action('pre_get_posts', 'wpsc_generate_product_query') ) // see if wpsc_generate_product_query is currently hooked in
                add_action( 'pre_get_posts', '__return_null', $priority ); // and give it a companion callback to avoid $wp_filter corruption
}