browser_name = navigator.appName;
browser_version = parseFloat(navigator.appVersion); 

if (browser_name == "Netscape" && browser_version >= 3.0) { roll = 'true'; }
else if (browser_name == "Microsoft Internet Explorer" && browser_version >= 4.0) { roll = 'true'; }
else { roll = 'false'; }

<!-- Carga das imagens se o navegador suportar o evento -->
	
	if (roll == 'true') {

	var listaimg = new Array (
	"images/vago.gif",
	"images/menu_bot_on.gif"
	);

	var imagens = new Array();
	
	var count;
	if (document.images)
	for (count=0; count<listaimg.length; count++)
	{imagens[count]=new Image(); imagens[count].src=listaimg[count];}
	}

	function msover1(img,ref) { if (roll == 'true') { document.images[img].src = ref; } }
	function msout1(img,ref) { if (roll == 'true') { document.images[img].src = ref; } }