XMLHttpRequest() in drupal

Does XMLHttpRequest(); not function properly in Drupal? I'm trying to use ajax to change things in my theme (instead of AHAH, long story, but I need straight ajax in the theming). I set up an ajax call, and none of the XMLHttpRequest functions are working properly?

var xmlhttp;

function showUser(str)
{
xmlhttp=GetXmlHttpObject();
if (xmlhttp==null)
{
alert ("Browser does not support HTTP Request");
return;
}

var url="getuser.php";
url=url+"?q=";
url=url+"&sid="+Math.random();

xmlhttp.onreadystatechange=stateChanged;

xmlhttp.open("GET",url,true);
xmlhttp.send(null);

}

function stateChanged()
{

if (xmlhttp.readyState==4)
{

document.getElementById("mytitle").innerHTML=xmlhttp.responseText;
}
}

function GetXmlHttpObject()
{
if (window.XMLHttpRequest)
{
// code for IE7+, Firefox, Chrome, Opera, Safari
return new XMLHttpRequest();
}
if (window.ActiveXObject)
{
// code for IE6, IE5
return new ActiveXObject("Microsoft.XMLHTTP");
}
return null;
}