If this file is remotely available to you, then you should be using the HTTP API.
$response = wp_remote_request(
'http://xml.prisguide.no/productExport.php?productId=151690'
,array(
'ssl_verify' => true // If the request isn't working, try it with `false`
)
);
You then simply go and catch the response and check if it’s an error:
if ( is_wp_error( $response ) )
// don't expose error to other users than admin
return current_user_can( 'manage_options' ) ? $response->get_error_message() : '';
Then you go and extract the content.
$content = trim( wp_remote_retrieve_body( $response ) );
$content = new SimpleXMLElement( $content );
Now do whatever you need with your retrieved data.