var agt=navigator.userAgent.toLowerCase();
var is_major = parseInt(navigator.appVersion);

var is_ie     = ((agt.indexOf("msie") != -1) && (agt.indexOf("opera") == -1));
var is_mac    = (agt.indexOf("mac")!=-1);
var is_ie5    = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.0")!=-1) );
var is_ie5_5    = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.5")!=-1) );