how to redirect "404 the requested product does not exist"

[jiyuu19th]

first of all, sorry for my poor English. I upgrade joomla 1.5.xx+vm 1.xx+sh404sef to joomla 2.5.6+vm2.0.6 (not use sh404sef)
some url got hxxp://www.mywebsite.com/?error=404     "404 the requested product does not exist"
can i redirect this error to homepage or 404 custom article page?

I try to copy error.php from /template/system , edit URL and phase to /template/my template
but it not working for me

Code: [Select]

defined( '_JEXEC' ) or die( 'Restricted access' );
if (($this->error->code) == '404') {
header('Location: http://www.mywebsite.com/404');
exit;
}
?>