I notice with IE6.0 : Microsoft.XMLHTTP don't take care at charset and work only with utf-8 encoded responses, while Msxml2.XMLHTTP seems, like XMLHttpRequest, respect the charset if it is done ( "Content-Type: text/html; charset=ISO-8859-1" in PHP). It seems explain your difficulties with jQuery.1.2.3 witch dispose (lines 2663) for ajax :
var xml = window.ActiveXObject ? new ActiveXObject("Microsoft.XMLHTTP") : new XMLHttpRequest(); while Peter Paul Kock (http://www.quirksmode.org/js/xmlhttp.html) recommends the reverse order and other ActiveXObjects : var XMLHttpFactories = [function () {return new XMLHttpRequest()}, function () {return new ActiveXObject("Msxml2.XMLHTTP")}, function () {return new ActiveXObject("Msxml3.XMLHTTP")}, function () {return new ActiveXObject("Microsoft.XMLHTTP")}]; Thanks