this.Show=function(title, msg, framesrc, w, h)
{
var iWidth = document.documentElement.clientWidth;
var iHeight = document.documentElement.clientHeight+document.documentElement.scrollTop*2;
var bgObj = document.createElement("div");
bgObj.style.cssText = "position:absolute;left:0px;top:0px;width:"+iWidth+"px;height:"+Math.max(document.body.clientHeight, iHeight)+"px;filter:Alpha(Opacity=30);opacity:0.3;background-color:#000000;z-index:101;";
document.body.appendChild(bgObj);

var msgObj=document.createElement("div");
msgObj.style.cssText = "position:absolute;font:11px '宋体';top:"+(iHeight-h)/2+"px;left:"+(iWidth-w)/2+"px;width:"+w+"px;height:"+h+"px;text-align:center;border:1px solid "+this.bordercolor+";background-color:"+this.bgcolor+";padding:1px;line-height:22px;z-index:102;";
document.body.appendChild(msgObj);

var table = document.createElement("table");
msgObj.appendChild(table);
table.style.cssText = "margin:0px;border:0px;padding:0px;";
table.cellSpacing = 0;
var tr = table.insertRow(-1);
var titleBar = tr.insertCell(-1);
titleBar.style.cssText = ";width:"+(w-84)+"px;height:"+this.titleheight+"px;text-align:left;padding:3px;margin:0px;font:bold 13px '宋体';color:"+this.titlecolor+";cursor:move;background-color:" + this.titlebgcolor;
titleBar.style.paddingLeft = "10px";
titleBar.innerHTML = title;
var moveX = 0;
var moveY = 0;
var moveTop = 0;
var moveLeft = 0;
var moveable = false;
var docMouseMoveEvent = document.onmousemove;
var docMouseUpEvent = document.onmouseup;

titleBar.onmousedown = function(){
var evt = getEvent();
moveable = true;
moveX = evt.clientX;
moveY = evt.clientY;
moveTop = parseInt(msgObj.style.top);
moveLeft = parseInt(msgObj.style.left);

document.onmousemove = function(){
if (moveable)
{
var evt = getEvent();
var x = moveLeft + evt.clientX - moveX;
var y = moveTop + evt.clientY - moveY;
if ( x > 0 &&( x + w < iWidth) && y > 0 && (y + h < iHeight) )
{
msgObj.style.left = x + "px";
msgObj.style.top = y + "px";
}
}
};

document.onmouseup = function (){
if (moveable)
{
document.onmousemove = docMouseMoveEvent;
document.onmouseup = docMouseUpEvent;
moveable = false;
moveX = 0;
moveY = 0;
moveTop = 0;
moveLeft = 0;
}
};
}

var closeBtn = tr.insertCell(-1);
closeBtn.style.cssText = "cursor:pointer; padding:2px;background-color:" + this.titlebgcolor;
closeBtn.innerHTML = "×关闭窗口";
closeBtn.onclick = function(){
document.body.removeChild(bgObj);
document.body.removeChild(msgObj);
}
var msgBox = table.insertRow(-1).insertCell(-1);
msgBox.style.cssText = "font:10pt '宋体';";
msgBox.colSpan = 2;

if(framesrc != "")
{
msg = "";
}
msgBox.innerHTML = ""+msg+"";

if(document.getElementById("frmAlertWin") != null)
{
document.getElementById("frmAlertWin").src = framesrc;
}

function getEvent(){
return window.event || arguments.callee.caller.arguments[0];
}
}
}

调用方式
var msgbox=new MessageBox();
msgbox.Show('站内公告',"小小提示","",500,220);///参数:标题,内容(提示为连接页面时可为空),连接页面地址(使用文字是可为空),宽度,高度
要注意的是如果在页面加载时就要弹出提示 请将调用代码放在body标记以后 否则在IE8下回出现错误