if ((navigator.appName == "Netscape" && parseInt(navigator.appVersion) >= 3) 
|| (navigator.appName == "Microsoft Internet Explorer" && parseInt(navigator.appVersion) >= 4))
{rollOvers = 1;} else {rollOvers = 0;} 

function switchOn(imgName) { 
if (rollOvers){ document [imgName].src = "/images/nav/" + imgName + "_on.gif";}
} 

function switchOff(imgName) { 
if (rollOvers){document [imgName].src = "/images/nav/" + imgName + "_off.gif";}
}