do following changes in ur code
<asp:TextBox ID="txtCaptcha" runat="server"></asp:TextBox>
<br />
<asp:Button ID="btnTryNew" runat="server" Text="TryNew" />
<asp:Button ID="btnVerify" runat="server" Text="Verify" OnClick="btnVerify_Click" ValidationGroup="Captcha" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="*Required" ValidationGroup="Captcha" ControlToValidate = "txtCaptcha"></asp:RequiredFieldValidator>