UserId of the Calling User in MS CRM 4.0

function GetCurrentUserInfo() {
var xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
xmlhttp.open("POST", "/mscrmservices/2007/crmservice.asmx", false);
xmlhttp.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
xmlhttp.setRequestHeader("SOAPAction", "http://schemas.microsoft.com/crm/2007/WebServices/Execute");
var soapBody = "<soap:Body>" +
"<Execute xmlns='http://schemas.microsoft.com/crm/2007/WebServices'>" +
"<Request xsi:type='WhoAmIRequest' />" +
"</Execute></soap:Body>";
var soapXml = "<soap:Envelope " +
"xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/' " +
"xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' " +
"xmlns:xsd='http://www.w3.org/2001/XMLSchema'>";
soapXml += GenerateAuthenticationHeader();
soapXml += soapBody;
soapXml += "</soap:Envelope>";
xmlhttp.send(soapXml);
xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
xmlDoc.async = false;
xmlDoc.loadXML(xmlhttp.responseXML.xml);
var userid = xmlDoc.getElementsByTagName("UserId")[0].childNodes[0].nodeValue;

return userid;
}