做了个AJAX验证，怎么只认IE，换上别的浏览器就不认，怎么办？

怎么样做能自动判断游览器？
新手上路，请多指教！

我的代码如下：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

<title>个人信息查询</title>



<script type="text/javascript">

<!--

function Initxmlhttp_request()

{

var xmlhttp_request=false;

try {

xmlhttp_request = new ActiveXObject("Msxml2.XMLHTTP");

} catch (e) {

try {

xmlhttp_request = new ActiveXObject("Microsoft.XMLHTTP");

} catch (e) {

xmlhttp_request = false;

}

}

if (!xmlhttp_request && typeof XMLHttpRequest!='undefined') {

xmlhttp_request = new XMLHttpRequest();

}

return xmlhttp_request;

}



function Onsearch()

{

var sinfo = document.getElementById("sinfo");

var msgs = document.getElementById("msgs");

msgs.innerHTML = "正在搜索...";

var f = document.form1;

var userselects = f.selects.options(f.selects.selectedIndex).value;

var userkeyword = f.keyword.value;



//接收表单的URL地址

var url = "search.php";



//POST的值

var postStr = "selects="+ userselects +"&keyword="+ userkeyword ;

var xmlhttp_request = Initxmlhttp_request();

xmlhttp_request.open("POST", url, true);



xmlhttp_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");



xmlhttp_request.send(postStr);



xmlhttp_request.onreadystatechange = function() {



if (xmlhttp_request.readyState == 4 && xmlhttp_request.status == 200) {

msgs.innerHTML = "";

sinfo.innerHTML = xmlhttp_request.responseText;

}

}

}



//-->

</script>

</head>


<body>

<table width="578" border="0" align="center" cellpadding="0" cellspacing="0">

<tr>

<td height="24" colspan="2" align="center" valign="middle" bgcolor="#4D4D4D">个人信息查询</td>

</tr>

<tr>

<td width="172" height="6" align="center" bgcolor="#202020"></td>

<td width="404" bgcolor="#202020"></td>

</tr>

<tr>

<td height="190" colspan="2" align="center" valign="top"><div id="searchmain">

<div align="left">请选择查询方式并填写查询信息 </div>

<form id="form1" name="form1" method="post" action="">

<label>

<select name="selects" id="selects">

<option value="name">按姓名</option>

<option value="sid">按学号</option>

</select>

<input name="keyword" type="text" id="keyword" />

<input name="Submit" type="button" onclick="Onsearch()" value="查询" />

</label>

</form>



<div id="msgs"></div>

</div>

<div id="sinfo"></div></td>

</tr>

<tr>

<td height="20" colspan="2" align="center" valign="top" ></td>

</tr>

</table>

</body>

</html>