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="http://www.al-stroy.ru/image/md_1.gif";
a2=new   Image;a2.src="http://www.al-stroy.ru/image/md_2.gif";
}