IE7浏览器无法运行AJax代码

原来我使用的是IE6，运行AJax一切正常，现升级到IE7后，却无法运行了，据说IE7内置了XMLHttpRequest对象，请问代码应该如何修改才能支持IE7!
原来的代码如下：!

var ReqTypeList;!

function GetList(){!
if (window.XMLHttpRequest){!
ReqTypeList = new XMLHttpRequest();!
}!
else if (window.ActiveXObject){!
ReqTypeList = new ActiveXObject("MSXML2.XMLHttp");!
}!
else if (window.ActiveXObject){!
ReqTypeList = new ActiveXObject("Microsoft.XMLHttp");!
}!

ReqTypeList.open("GET","XXXXXXXXXX.asp",true);!
ReqTypeList.onreadystatechange = GetListCallBack;!
ReqTypeList.send(null);!
}!

function GetListCallBack(){!
if (ReqTypeList.readystate == 4){!
if (ReqTypeList.status == 200){!
document.all.TypeList.innerHTML = ReqTypeList.responseText;!
}!
}!
}!


!

ie7没有用过,也只是听说有了那个对象.
你看看进到这个if里没?
if (window.XMLHttpRequest){alert("in");//弄个alert看看进来没
ReqTypeList = new XMLHttpRequest();
}

进来学习学习下``