This may help you:

How to Pass Messages and Actions between Server and Client
http://usableasp.net/DeveloperPage.aspx?page=Articles/HowTo/HowToPassMessagesBet
weenServerAndClient.htm

this can only be done in javascript. you need to understand the web
model. the browser request a page, asp.net process the request and sends
back an html document. the doc header can contain a redirect command,
but there is no open window command. your return document can contain
inline javascript that is executed when the browser renders the page.

<script>window.alert('hi');</script>

in the old days before popup blockers, the inline javascript could open
a window.

-- bruce (sqlwork.com)

You could always display the error, then auto-redirect after x
seconds, or a user click

in side js block...
function redirTimer( sURL, iSecondsDelay){
    self.setTimeout("self.location.href='" + sURL + "';",
iSecondsDelay);
    //show(false,'testJava');
}

or

open a new window after page load (pop up blockers could be
problematic like bruce mentioned.

function popWin1(url, thePreference)
{
  // Create offset
      if (document.all) {
             xMax = screen.width;
        yMax = screen.height;
      }
    else {
             if (document.layers) {
               xMax = window.outerWidth;
            yMax = window.outerHeight;
             }
            else {
                xMax = 400;
            yMax=480;
             }
      }
      var xOffset = (xMax - 586)/2;
    var yOffset = (yMax - 700)/2;

      thePreference = thePreference + ',screenX='+xOffset
+',screenY='+yOffset+',top='+yOffset+',left='+xOffset
+'resizable=true,scrollbars=yes';

  // alert("Preference = " + thePreference);
    if (typeof(popupWin) != "object"){
        popupWin = window.open(url, 'window2', thePreference);
    }
    if (popupWin.closed){
        popupWin = window.open(url, 'window2', thePreference);
    }
    if( popupWin.opener == window) {
        popupWin.focus();
        return;
    }
    return;
}