function AlertMsg(){
   var sWidth,sHeight;
   sWidth = window.screen.availWidth-20;
   if(window.screen.availHeight > document.body.scrollHeight){
    sHeight = window.screen.availHeight; 
   }else{
    sHeight = document.body.scrollHeight;   
   }
   var maskObj = document.createElement("div");
   maskObj.setAttribute('id','BigDiv');
   maskObj.style.position = "absolute";
   maskObj.style.top = "0";
   maskObj.style.left = "0";
   maskObj.style.background = "#000";
   maskObj.style.filter = "Alpha(opacity=45);";
   maskObj.style.opacity = "0.3";
   maskObj.style.width = sWidth + "px";
   maskObj.style.height = sHeight + "px";
   maskObj.style.zIndex = "10000";
   document.body.appendChild(maskObj);

        var MyDiv =document.getElementById("MyDiv");
   var MyDiv_w = getStyle(MyDiv,"width");
   var MyDiv_h = getStyle(MyDiv,"height");
     
   MyDiv_w = parseInt(MyDiv_w);
   MyDiv_h = parseInt(MyDiv_h);

   var width = pageWidth(); 
   var height = pageHeight();
   var left = leftPosition();
   var top = topPosition();

   var Div_topposition = top + (height / 2) - (MyDiv_h / 2);
   var Div_leftposition = left + (width / 2) - (MyDiv_w / 2);

   MyDiv.style.left = Div_leftposition + "px";
   MyDiv.style.top = Div_topposition + "px";
   MyDiv.style.display = "block";
}

function CloseDiv(){
   var Bigdiv = document.getElementById("BigDiv");
   var Mydiv = document.getElementById("MyDiv");
   document.body.removeChild(Bigdiv); 
   Mydiv.style.display="none";
}

window.onscroll = window_onscroll;
function window_onscroll(){
   var MyDiv =document.getElementById("MyDiv");
   var MyDiv_h = getStyle(MyDiv,"height");
   MyDiv_h = parseInt(MyDiv_h);
   var height = pageHeight();
   var top = topPosition();
   var Div_topposition = top + (height / 2) - (MyDiv_h / 2);
   MyDiv.style.top = Div_topposition + "px";
}


function pageWidth(){
    return window.innerWidth != null ? window.innerWidth : document.documentElement && document.documentElement.clientWidth ? document.documentElement.clientWidth : document.body != null ? document.body.clientWidth : null;
}


function pageHeight(){
    return window.innerHeight != null? window.innerHeight : document.documentElement && document.documentElement.clientHeight ? document.documentElement.clientHeight : document.body != null? document.body.clientHeight : null;
}

function topPosition(){
     return typeof window.pageYOffset != 'undefined' ? window.pageYOffset : document.documentElement && document.documentElement.scrollTop ? document.documentElement.scrollTop : document.body.scrollTop ? document.body.scrollTop : 0;
}

function leftPosition(){
    return typeof window.pageXOffset != 'undefined' ? window.pageXOffset : document.documentElement && document.documentElement.scrollLeft ? document.documentElement.scrollLeft : document.body.scrollLeft ? document.body.scrollLeft : 0;
}

function getStyle(elem, name){
    if(elem.style[name])
     return elem.style[name];
    else if(elem.currentStyle)
     return elem.currentStyle[name];
    else if(document.defaultView && document.defaultView.getComputedStyle){ //w3c
     name = name.replace(/([A-Z])/g,"-$1");
     name = name.toLowerCase();
    
     var s = document.defaultView.getComputedStyle(elem,"");
     return s && s.getPropertyValue(name);
    } else
     return null
}