var nom = navigator.appName.toLowerCase();
var agt = navigator.userAgent.toLowerCase();
var is_major  = parseInt(navigator.appVersion);
var is_minor  = parseFloat(navigator.appVersion);
var is_ie     = (agt.indexOf("msie") != -1);
var is_ie4up  = (is_ie && (is_major >= 4));
var is_nav    = (nom.indexOf('netscape')!=-1);
var is_nav4   = (is_nav && (is_major == 4));
var is_mac    = (agt.indexOf("mac")!=-1);
var is_gecko  = (agt.indexOf('gecko') != -1);