//自动关闭提示框
function Alert(str) {
    var msgw,msgh,bordercolor;
    msgw=350;//提示窗口的宽度
    msgh=80;//提示窗口的高度
    titleheight=25 //提示窗口标题高度
    bordercolor="#336699";//提示窗口的边框颜色
    titlecolor="#99CCFF";//提示窗口的标题颜色
    var sWidth,sHeight;
    //获取当前窗口尺寸
    sWidth = document.body.offsetWidth;
    sHeight = document.body.offsetHeight;
//    //背景div
    var bgObj=document.createElement("div");
    bgObj.setAttribute('id','alertbgDiv');
    bgObj.style.position="absolute";
    bgObj.style.top="0";
    bgObj.style.background="#E8E8E8";
    bgObj.style.filter="progid:DXImageTransform.Microsoft.Alpha(style=3,opacity=25,finishOpacity=75";
    bgObj.style.opacity="0.6";
    bgObj.style.left="0";
    bgObj.style.width = sWidth + "px";
    bgObj.style.height = sHeight + "px";
    bgObj.style.zIndex = "10000";
    document.body.appendChild(bgObj);
    //创建提示窗口的div
    var msgObj = document.createElement("div")
    msgObj.setAttribute("id","alertmsgDiv");
    msgObj.setAttribute("align","center");
    msgObj.style.background="white";
    msgObj.style.border="1px solid " + bordercolor;
    msgObj.style.position = "absolute";
    msgObj.style.left = "50%";
    msgObj.style.font="12px/1.6em Verdana, Geneva, Arial, Helvetica, sans-serif";
    //窗口距离左侧和顶端的距离 
    msgObj.style.marginLeft = "-225px";
    //窗口被卷去的高+（屏幕可用工作区高/2）-150
    msgObj.style.top = document.body.scrollTop+(window.screen.availHeight/2)-150 +"px";
    msgObj.style.width = msgw + "px";
    msgObj.style.height = msgh + "px";
    msgObj.style.textAlign = "center";
    msgObj.style.lineHeight ="25px";
    msgObj.style.zIndex = "10001";
    document.body.appendChild(msgObj);
    //提示信息标题
    var title=document.createElement("h4");
    title.setAttribute("id","alertmsgTitle");
    title.setAttribute("align","left");
    title.style.margin="0";
    title.style.padding="3px";
    title.style.background = bordercolor;
    title.style.filter="progid:DXImageTransform.Microsoft.Alpha(startX=20, startY=20, finishX=100, finishY=100,style=1,opacity=75,finishOpacity=100);";
    title.style.opacity="0.75";
    title.style.border="1px solid " + bordercolor;
    title.style.height="18px";
    title.style.font="12px Verdana, Geneva, Arial, Helvetica, sans-serif";
    title.style.color="white";
    title.innerHTML="提示信息";
    document.getElementById("alertmsgDiv").appendChild(title);
    //提示信息
    var txt = document.createElement("p");
    txt.setAttribute("id","msgTxt");
    txt.style.margin="16px 0";
    txt.innerHTML = str;
    document.getElementById("alertmsgDiv").appendChild(txt);
    //设置关闭时间
    window.setTimeout("closewin()",2000); 
}
function closewin() {
    document.body.removeChild(document.getElementById("alertbgDiv"));
    document.getElementById("alertmsgDiv").removeChild(document.getElementById("alertmsgTitle"));
    document.body.removeChild(document.getElementById("alertmsgDiv"));
}

?? 使用方法 直接调用Alert('提示信息')即可，如下：

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???<script type="text/javascript">

     function form_check(){
     if(document.form1.user.value==""){Alert("请输入用户名！");document.form1.user.focus();return (false);}
     if(document.form1.password.value==""){Alert("请输入密码！");document.form1.password.focus();return (false);}
     }
</script>

<form name="form1" onsubmit="return form_check();" method="post" action="login.asp"> 
用户名： <input name="user" type="text" id="user"/>
密码： <input name="password" type="password" id="password" />
<INPUT id=btnEnter type=image src="images/login.gif" name=btnEnter> 

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