Hi,
You can create this effect using simple divs as well.
1.Create a div1 with width and height same as your page.
2.Put your popup in a div2.
3.Set your z-index of div such that the popup div2 is above the div1.
4.Set the opacity,filter to 50% for the div1.
5.Make all this divs visible in your page load.
Code samples:
div1 code:
<div id="screenhide" visible="false" runat="server" style="background-color:#fff;width:100%; height:100%; position: absolute; top:0; left:0; z-index: 5;opacity:0.8;filter:alpha(opacity=80)">
</div>
Hope this helps.