prova così:

codice:
<script language="JavaScript">
<!--
browser_name = navigator.appName;
browser_version = parseFloat(navigator.appVersion);
if (browser_name == "Microsoft Internet Explorer") {
 if (browser_version <= 4.3) {
alert("ho letto versione: '"+browser_version+"'");
parent.location.href='old.html';
}
 else if (screen.width!=800) parent.location.href='_1/index_1.html';
}
//-->
</script>
ciao