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;
}

function Swap(img1,ref1) {
        if (roll == true) {
                document.images[img1].src = ref1;
        }
}