For a stylish popup in .net use code.....
<asp:Panel runat="server" ID="pnlPopUp" style="display:none;">
<div style="position: absolute; width: 100%; background: #333; height: 820px; top: 0;
left: 0; opacity: 0.6; filter: alpha(opacity=60);"></div>
<div style="position: absolute; box-shadow: 0 0 3px 3px #333; top: 30%; background: #fff;
left: 25%; z-index: 1000; min-width:450px;">
<!-- Add your contents here -->
</div>
</div>
</panel>
at the page load add code as
pnlPopUp.Style.Add("display", "block");
or if u r using div instead of panel call script at pageload as
Page.RegisterStartupScript("s", "<script>functionName()</script>");
and script is ..
function functionName()
{
var a= document.getElementById("pnlPopUp");
a.style.display="block";
}
May this help you.....