function myPopupRelocate() 
    {
        var scrolledX, scrolledY;
        if( self.pageYOffset ) 
        {
            scrolledX = self.pageXOffset;
            scrolledY = self.pageYOffset;
        } 
        else if( document.documentElement && document.documentElement.scrollTop ) 
        {
            scrolledX = document.documentElement.scrollLeft;
            scrolledY = document.documentElement.scrollTop;
        } 
        else if( document.body ) 
        {
            scrolledX = document.body.scrollLeft;
            scrolledY = document.body.scrollTop; 
        }

        var centerX, centerY;
        if( self.innerHeight ) 
        {
            centerX = self.innerWidth;
            centerY = self.innerHeight;
        } 
        else if( document.documentElement && document.documentElement.clientHeight ) 
        {
            centerX = document.documentElement.clientWidth;
            centerY = document.documentElement.clientHeight;
        } 
        else if( document.body ) 
        {
            centerX = document.body.clientWidth;
            centerY = document.body.clientHeight;
        }

         var leftOffset = scrolledX + (centerX - 250) / 2;
         var topOffset = scrolledY + (centerY - 200) / 2;

         document.getElementById("mypopup").style.top = topOffset + "px";
         document.getElementById("mypopup").style.left = leftOffset + "px";
    }

    function fireMyPopup() 
    {
        myPopupRelocate();
        document.getElementById("mypopup").style.display = "block";
        document.body.onscroll = myPopupRelocate;
        window.onscroll = myPopupRelocate;
    }