IE7 javascript createDocument issue :(

Hi Experts,

I have a problem where the below javascript wont work with IE7 :( I keep getting:
"Object doesn;t support this property or method"

Works fine in IE6 and Firefox. You'll see the alerts I've inserted to debug it. The problem line is:
"rt = document.implementation.createDocument('','',null);"

Any help will be great. Thanks.

Code:
---------------------------------------------------------------------
function SoapService()
{      
      //branch for Mozilla
      if (window.XMLHttpRequest)
      {
        this.XmlHttp = new XMLHttpRequest();
            if (this.XmlHttp.overrideMimeType) {
                  this.XmlHttp.overrideMimeType('text/xml');
            }

        this.Version=0;
    }
    // branch for IE6 / Windows ActiveX version
    else if (window.ActiveXObject)
    {
            this.XmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
            this.Version=1;
      }
      this.callService=_callService;
}
function _callService(func,params,async)
{
try{
      this.XmlHttp.open("POST", params[0], async);
      this.XmlHttp.setRequestHeader('Content-Type','text/xml; charset=UTF-8');
      this.XmlHttp.setRequestHeader('SOAPAction', '"'+params[1]+'"');
      var rt;
      if(async)
      {
            var XmlHttpAsync=this.XmlHttp;
            var VersionAsync=this.Version;
            this.XmlHttp.onreadystatechange =function()
            {
                  if(XmlHttpAsync.readyState != 4)return;
                  //alert(XmlHttpAsync.responseText);
                  if(VersionAsync==0)
                  {
                        rt = document.implementation.createDocument('','',null);
                        rt=XmlHttpAsync.responseXML;
                  }
                  else if(VersionAsync==1)
                  {
                        rt= XmlHttpAsync.responseXML.documentElement;
                  }
                  func(rt);
                  return true;

            }
      }
      this.XmlHttp.send(params[2]);
      if(!async)
      {
            //alert(this.XmlHttp.responseText);
            if(this.Version==0)
            {
                  alert('LOOK AT THIS!! START');
                  rt = document.implementation.createDocument('','',null);
                  alert('LOOK AT THIS!! MID');
                  rt=this.XmlHttp.responseXML;
                  alert('LOOK AT THIS!! FIN');
            }
            else if(this.Version==1)
            {
                  rt= this.XmlHttp.responseXML.documentElement;
            }
            func(rt);
            return true;
      }
}
catch(e){
            alert(e.description);
            return false;
}
}
function XmlRegExp(IrregExp)
{
      var strXml=IrregExp.toString();
      return strXml.replace(/\&/g,"&amp;").replace(/</g,"&lt;").replace(/>/g,"&gt;");
}