string url = "window_page";
string fullURL = "window.open('" + url + "' );";
ScriptManager.RegisterStartupScript(this, typeof(string), "OPEN_WINDOW", fullURL, true);
Improved Solution
string fullURL = "window.open(' http://www.google.de ' , 'width=1003,status=yes,toolbar=no,menubar=no,location=no,scrollbars=yes,resizable=no,titlebar=no' );";
ScriptManager.RegisterStartupScript(this, typeof(string), "OPEN_WINDOW", fullURL, true);