Ajax Script not working in IE (window.XMLHttpRequest)

Hi,

I have written following code in ajax.

This is a javascript function called on onchange event of a button.

This code works fine in firefox and chrome but does not work in IE8.0+
the values don't get displayed.

plz suggest me if there is any change.

thnx in advance.

<script type="text/javascript">
    
function showUser(str) {
  if (str=="") {
    document.getElementById("userDetailsShownHere").innerHTML="";
    return;
  }
  if (window.XMLHttpRequest) {
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
  } else { // code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
  xmlhttp.onreadystatechange=function() {
    if (xmlhttp.readyState==4 && xmlhttp.status==200) {
      document.getElementById("userDetailsShownHere").innerHTML=xmlhttp.responseText;
      document.getElementById("btn").name=str;
        showGift(str);
    }
  }
  xmlhttp.open("GET","getuser.php?username="+str,true);
  xmlhttp.send();
}
//This code will execute on button click
function showGift(str) {
    var life = document.getElementById("lifeLabel").value;
    var number = parseInt(life, 0);
    if(number == 55){      
        if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
      } else { // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
      xmlhttp.onreadystatechange=function() {
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            
          document.getElementById("userDetailsShownHere").innerHTML=xmlhttp.responseText;
         document.getElementById("btn").name=str;
       document.getElementById("btn").style.display="inline";
       
        }
      }
      xmlhttp.open("GET","getuser.php?username="+str,true);
      xmlhttp.send();
    }
    else if(number > 0){
      document.getElementById("noLife").style.display="none";
        number = number - 1;

      if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
      } else { // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
       xmlhttp.onreadystatechange=function() {
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {
          document.getElementById("userDetailsShownHere").innerHTML=xmlhttp.responseText;
          document.getElementById("btn").name=str;
        document.getElementById("btn").style.display="none";
         if(document.getElementById("cometomorrow") || document.getElementById("win")){
            document.getElementById("btn").style.display="none";
            console.log("hide");
          }else{
            setTimeout("showButton()", 1000);
          }
        }
      }
      xmlhttp.open("GET","getgift.php?username="+str+"&loop="+number,true);
      xmlhttp.send();
    
    }
    else {
    document.getElementById("userDetailsShownHere").innerHTML="";
        document.getElementById("noLife").style.display="inline";
    document.getElementById("btn").style.display="none";
    }
}

</script>