Come richiami attualmente il controllo?

presumo:

<form action="pagina.asp" onsubmit="MM_validateForm(); return MM_returnValue;">

cambia:
<form action="pagina.asp"
target="pippopluto"
onsubmit="MM_validateForm(); if(MM_returnValue) window.open('','pippopluto');return MM_returnValue;">

ciao