重写alert 方法（我胡汉三又回来了）

 window.alert = function (txt) {
     var shield = document.createElement("DIV");
     shield.id = "shield";
     shield.style.position = "absolute";
     shield.style.left = "0px";
     shield.style.top = "0px";
     shield.style.width = "100%";
     shield.style.height = document.body.scrollHeight + "px";
     shield.style.background = "#333";
     shield.style.textAlign = "center";
     shield.style.zIndex = "10000";
     shield.style.filter = "alpha(opacity=50);";
     shield.style.opacity = 0.5;
     var alertFram = document.createElement("DIV");
     alertFram.id = "alertFram";
     alertFram.style.position = "absolute";

     alertFram.style.marginTop = "-" + (document.body.scrollHeight - document.body.scrollTop - (window.screen.availHeight / 2) + 180) + "px";
     alertFram.style.marginLeft = (document.body.clientWidth / 2 - 200) + "px";
     alertFram.style.width = "400px";
     alertFram.style.height = "180px";
     alertFram.style.background = "#4c4c4c";
     alertFram.style.textAlign = "center";
     alertFram.style.lineHeight = "180px";
     alertFram.style.zIndex = "10001";
     strHtml = "<ul style=\"list-style:none;margin:0px;padding:10px;width:380px; height:160px;\">\n";
     strHtml += " <li style=\"background:#fff;text-align:left;padding-left:15px;font-size:14px;font-weight:bold;height:39px;line-height:39px;border-bottom:1px solid #c8c8c8;\">温馨提示<span style=\" float:right; margin-right:15px; font-size:15px; cursor:pointer;\" onclick=\"doOk()\">×</span></li>\n";
     strHtml += " <li style=\"background:#fff;text-align:center;font-size:13px;height:80px;line-height:80px;\">" + txt + "</li>\n";
     strHtml += " <li style=\"background:#fff;text-align:center;height:40px;line-height:40px;\"><input type=\"button\" style=\"width:78px; height:28px; border:1px #be1414 solid; color:#fff; font-size:14px; background:#e52828;\" value=\"确 定\" onclick=\"doOk()\" /></li>\n";
     strHtml += "</ul>\n";
     alertFram.innerHTML = strHtml;
     document.body.appendChild(alertFram);
     document.body.appendChild(shield);
     var c = 0;
     this.doAlpha = function () {
         if (c++ > 20) { clearInterval(ad); return 0; }
         shield.style.filter = "alpha(opacity=" + c + ");";
     }
     var ad = setInterval("doAlpha()", 5);
     this.doOk = function () {
         alertFram.style.display = "none";
         shield.style.display = "none";
     }
     alertFram.focus();
     document.body.onselectstart = function () { return false; };
 };

兼容性还没测试  哈哈 ~ 各位借鉴哦