function createXHR() {
    if (typeof XMLHttpRequest !== 'undefined') {
        return new XMLHttpRequest(); // 现代浏览器
    } else if (typeof ActiveXObject !== 'undefined') {
        if (typeof arguments.callee.activeXString !== 'string') {
            var versions = ['MSXML2.XMLHttp.6.0', 'MSXML2.XMLHttp.3.0', 'MSXML2.XMLHttp'],
                i, len;
            for (i = 0, len = versions.length; i < len; i++) {
                try {
                    new ActiveXObject(versions[i]);
                    arguments.callee.activeXString = versions[i];
                    break;
                } catch (ex) {
                    // 跳过
                }
            }
        }
        return new ActiveXObject(arguments.callee.activeXString); // IE6/7
    } else {
        throw new Error('No XHR object available.');
    }
}

var xhr = createXHR();
xhr.open('get', 'example.php?rand=' + Math.random(), true); // 添加随机参数
xhr.send(null);

xhr.setRequestHeader("If-Modified-Since", "0");
xhr.setRequestHeader("Cache-Control", "no-cache");

function createCORSRequest(method, url) {
    var xhr = new XMLHttpRequest();
    if ('withCredentials' in xhr) {
        // 现代浏览器
        xhr.open(method, url, true);
    } else if (typeof XDomainRequest !== 'undefined') {
        // IE8/9
        xhr = new XDomainRequest();
        xhr.open(method, url);
    } else {
        // 不支持CORS
        xhr = null;
    }
    return xhr;
}

$.ajax({
    url: 'example.php',
    cache: false, // 防止缓存
    success: function(data) {
        // 处理数据
    }
});