new ActiveXObject("microsoft.xmldom")的兼容,该怎么处理

new ActiveXObject("microsoft.xmldom")的兼容
//验证用户的昵称
var   xmlUser=new   ActiveXObject( "microsoft.xmldom ");
var   timer;
function   chkUsername(username)
{
        if(username== " ")
        ……
        }        
        var   info   =   document.getElementById( 'info ');
        info.innerHTML= "检查用户名.... ";
        xmlUser.load( "CheckUserAction.do?username= "+username);
        LoadUserInfo();
}
function   LoadUserInfo()
{  
    window.clearTimeout(timer);
    if(xmlUser.readyState== "4 ")
    {
        ……    
    }
    else
    {
        ……
    }
}


请问一下这种情况下ie和firefox下如何兼容var   xmlUser=new   ActiveXObject( "microsoft.xmldom ");
谢谢！问题得到解决后马上结帖给分。

[解决办法]
function createAJAXObj() {
var A;
var msxmlhttp = new Array(
'Msxml2.XMLHTTP.5.0 ',
'Msxml2.XMLHTTP.4.0 ',
'Msxml2.XMLHTTP.3.0 ',
'Msxml2.XMLHTTP ',
'Microsoft.XMLHTTP ');

for (var i = 0; i < msxmlhttp.length; i++) {
try {
A = new ActiveXObject(msxmlhttp[i]);
} catch (e) {
A = null;
}
}

if(!A && typeof XMLHttpRequest != "undefined ")
A = new XMLHttpRequest();
if (!A)
A = false;
return A;
}