看如下的javascript脚本: 
if (window.XMLHttpRequest) { //Mozilla, Safari,...IE7 
alert('Mozilla, Safari,...IE7 ');
if(!window.ActiveXObject){ // Mozilla, Safari,...
alert('Mozilla, Safari');
} else {
alert('IE7');
}
} else {
alert('IE6');

  看看运行效果: 
<script language="javascript">
if (window.XMLHttpRequest) { //Mozilla, Safari,...IE7 
alert('Mozilla, Safari,...IE7 ');
if(!window.ActiveXObject){ // Mozilla, Safari,...
alert('Mozilla, Safari');
} else {
alert('IE7');
}
} else {
alert('IE6');
}
</script>