how to open a new window in ontextchanged in c# .net

code: 
<asp:TextBox ID="txtbox" runat="server".......AutoPostBack="true" OnTextChanged="txtserialnoseach_TextChanged"></asp:TextBox>

protected void txtserialnoseach_TextChanged(object sender, EventArgs e) {
if (ds.Tables[0].Rows.Count == 0) {
ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "alert('Not Available.');", true);
---- then open a new window ------- } else {
}
}