Make sure we have a fallback when XMLHttpRequest is manually disabled. Fixes #6298.

src/ajax.js

@@ -180,19 +180,10 @@ jQuery.extend({
 		password: null,
 		traditional: false,
 		*/
-		// Create the request object; Microsoft failed to properly
-		// implement the XMLHttpRequest in IE7 (can't request local files),
-		// so we use the ActiveXObject when it is available
 		// This function can be overriden by calling jQuery.ajaxSetup
-		xhr: window.XMLHttpRequest && (window.location.protocol !== "file:" || !window.ActiveXObject) ?
-			function() {
-				return new window.XMLHttpRequest();
-			} :
-			function() {
-				try {
-					return new window.ActiveXObject("Microsoft.XMLHTTP");
-				} catch(e) {}
-			},
+		xhr: function() {
+			return new window.XMLHttpRequest();
+		},
 		accepts: {
 			xml: "application/xml, text/xml",
 			html: "text/html",
@@ -695,6 +686,27 @@ jQuery.extend( jQuery.ajax, {
 
 });
 
+/*
+ * Create the request object; Microsoft failed to properly
+ * implement the XMLHttpRequest in IE7 (can't request local files),
+ * so we use the ActiveXObject when it is available
+ * Additionally XMLHttpRequest can be disabled in IE7/IE8 so
+ * we need a fallback.
+ */
+if ( window.ActiveXObject ) {
+	jQuery.ajaxSettings.xhr = function() {
+		if ( window.location.protocol !== "file:" ) {
+			try {
+				return new window.XMLHttpRequest();
+			} catch(e) {}
+		}
+
+		try {
+			return new window.ActiveXObject("Microsoft.XMLHTTP");
+		} catch(e) {}
+	};
+}
+
 // Does this browser support XHR requests?
 jQuery.support.ajax = !!jQuery.ajaxSettings.xhr();