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 >= 3.0) { roll = 'true'; } 
else { roll = 'false'; }  

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

if (roll == 'true') 
{ 
a1=new Image;a1.src="t1a.gif";
}