重写Alert

var isIE = navigator.appName.indexOf("Internet Explorer") != -1;
var oldAlert = window.alert;
window.alert = function (str) { 
var eSrc;
if (!isIE) {
eSrc = (document.all) ? window.event.srcElement : arguments[1];
} else {
eSrc = document.activeElement;
}
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.documentElement.clientHeight > document.documentElement.scrollHeight) ? document.documentElement.clientHeight : document.documentElement.scrollHeight) + "px";
shield.style.background = "#FFF000";
shield.style.textAlign = "center";
shield.style.zIndex = "10000";
shield.style.filter = "alpha(opacity=0)";
shield.style.opacity = 0;
var alertFram = document.createElement("DIV");
alertFram.id = "alertFram";
alertFram.style.position = "absolute";
alertFram.style.left = "50%";
alertFram.style.top = "50%";
alertFram.style.marginLeft = "-225px";
alertFram.style.marginTop = -75 + document.documentElement.scrollTop + "px";
alertFram.style.width = "450px";
alertFram.style.height = "150px";
alertFram.style.background = "#ccc";
alertFram.style.textAlign = "center";
alertFram.style.lineHeight = "150px";
alertFram.style.zIndex = "10001";
strHtml = "<ul style="list-style:none;margin:0px;padding:0px;100%"> ";
strHtml += " <li style="background:#DD828D;text-align:left;padding-left:20px;font-size:14px;font-weight:bold;height:25px;line-height:25px;border:1px solid #F9CADE;">[系统提示]</li> ";
strHtml += " <li style="background:#fff;text-align:center;font-size:12px;height:120px;line-height:120px;border-left:1px solid #F9CADE;border-right:1px solid #F9CADE;">" + str + "</li> ";
strHtml += " <li style="background:#FDEEF4;text-align:center;font-weight:bold;height:25px;line-height:25px; border:1px solid #F9CADE;"><input type="button" value="确 定" id="do_OK" onclick="doOk()" /></li> ";
strHtml += "</ul> ";
alertFram.innerHTML = strHtml;
document.body.appendChild(alertFram);
document.body.appendChild(shield);
this.setOpacity = function (obj, opacity) {
if (opacity >= 1) opacity = opacity / 100;
try { obj.style.opacity = opacity; } catch (e) { }
try {
if (obj.filters.length > 0 && obj.filters("alpha")) {
obj.filters("alpha").opacity = opacity * 100;
} else {
obj.style.filter = "alpha(opacity="" + (opacity * 100) + "")";
}
} catch (e) { }
}
var c = 0;
this.doAlpha = function () {
if (++c > 20) { clearInterval(ad); return 0; }
setOpacity(shield, c);
}
var ad = setInterval("doAlpha()", 1);
this.doOk = function () {
//alertFram.style.display = "none";
//shield.style.display = "none";
document.body.removeChild(alertFram);
document.body.removeChild(shield);
eSrc.focus();
document.body.onselectstart = function () { return true; }
document.body.oncontextmenu = function () { return true; }
}
document.getElementById("do_OK").focus();
eSrc.blur();
document.body.onselectstart = function () { return false; }
document.body.oncontextmenu = function () { return false; }
}

效果如下:

样式还可自己再改
}