PHP,Ajax & Browser Problem

Hi
I have written a PHP & ajax script which dynamically select a dependent dropdown select box. The script is running perfectly in Firefox but not in Internet Explorer & Safari. I have to run the script in all the browser perfectly. If there is any error in my script,please help me to solve it out.

<select name="category" size="1" class="body-text-grey" onchange="showDivision(this.value)">
<option selected="selected" value="blank">Choose a Category</option>
<?php
$res=mysql_query("select * from mbehub_categories where parent_id='0' order by id") or die (mysql_error());
while ($rw=mysql_fetch_array($res))
{

?>
<option value="<?php echo $rw['id'];?>"><?php echo $rw['name'];?></option>
<?php
}
?>
</select>

<tr>
<td height="22" align="left" valign="top" class="body-text-grey-2">Primary Area of Interest </td>
<td height="22" colspan="2" align="left" valign="top">
<select name="division" size="1" class="body-text-grey" id="txtDivision">
<option selected="selected">Select Area of Interest</option>
</select>


</td>
</tr>

HTML:

And the Ajax is :

// JavaScript Document
var xmlHttp;function showDivision(str)
{
xmlHttp=GetXmlHttpObject();
if (xmlHttp==null)
{
alert ("Browser does not support HTTP Request");
return;
}
var url="division.php";
url=url+"?q="+str;
url=url+"&sid="+Math.random();
xmlHttp.onreadystatechange=stateChanged;
xmlHttp.open("GET",url,true);
xmlHttp.send(null);
}function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById("txtDivision").innerHTML=xmlHttp.responseText;
}
}function GetXmlHttpObject()
{
var xmlHttp=null;
try
{
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e)
{
//Internet Explorer
try
{
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e)
{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
}
return xmlHttp;
}

Code (markup):

 

First line, missing ID:

<select name="category" size="1" class="body-text-grey" onchange="showDivision(this.value)">

<select name="category" size="1" class="body-text-grey" onchange="showDivision(this.value)" id="category">

// this refers to an ID, not a name?