var loggedInXML = new XML();
loggedInXML.ignoreWhite = true;

var _login:LoadVars = new LoadVars();
_login.Username = Name.text;
_login.Password = Pwd.text;
_login.sendAndLoad("theXML.asp",_login,"POST");
_login.onLoad = function(){
if(this.passed = "OK"){ // if login success, asp returns - &passed=OK&
loggedInXML.load("urlTo.xml"); // load the xml
} else { doError(); } // some error output
};

loggedInXML.onLoad = function(success:Boolean){
if (success) { /* parse the xml  */ }
};