AJAX problem: slow response using IE6 on Win2000 versus IE6 on XP

[Pugi!]

I have an AJAX application.
Using IE6, FF2.0, FF1.5, IE7 and Opera9 on WinXP the responses are
very quick. If the server returns an OK message the formù is removed
from screen. If not (because some required fields have no value, or
some fields have an invalid value) and error message is displayed
underneath the field..
Using FF1.5 and FF2.0 on Win2000 gives quick responses, but using IE6
on Win2000 there is a delay between alert('send info') and
alert('response from server') 15 till 20 seconds.
It is an AJAX-application, so I use a lot of XMLHttpRequests. For an
AJAX GET I don't notice any difference for the different browser and
OS combinations, same goes for a SUBMIT POST (where I direct
output(javascript) to a hidden iframe). The problem with the long
delay with an AJAX POST only occurs using IE6 on Win2000 . Is this a
known problem or should use different parameters for my AJAX POST ?

here I define url and strPos
....
alert('send info');
http.open('POST', url, true);
http.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded; charset=UTF-8");
http.setRequestHeader("Content-length", strPost.length);
http.setRequestHeader("Connection", "close");
http.onreadystatechange = function () {
if (http.readyState == 4) {
if(http.status == 200) {
alert('response from server')
var dom = http.responseXML;
var messg = dom.getElementsByTagName('message');
var target = document.getElementById('folder_' +
parentFolderID);
if (messg.length > 0) {
switch(messg[0].getAttribute('level')) {
case 'ok':
closeForm(frmNode);
refreshFolder(target);
break;
case 'warning':
case 'error':
closeForm(frmNode);
alert(messg[0].getAttribute('level') + ' : ' +
messg[0].getAttribute('text'))
break;
case 'notice':

errTarget.appendChild(document.createTextNode('Error: ' +
messg[0].getAttribute('text')));
break;
}
}
}
}
}
http.send(strPost);

Johan