[melky]

ты забыл подчёркнутое

window.onload = XmlHttp;

function XmlHttp(){
	var name = newfolder;
	xml = new XMLHttpRequest();
	var params = 'name=' + encodeURIComponent(name);
	xml.open("POST", 'post.php', true);
	xml.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
*!*xml.setRequestHeader("Content-length", params.length);*/!*
	xml.send(params);
}