I hope you will get the same problem which i faced when we use ASP.NET 2.0 validation controls with Ajax controls ( inside Update panel)  :'(

so in this article i'm going to discuss about the difference between the ASP.NET 2.0 and the Ajax validation controls.  

The difference between the ASP.NET 2.0 AJAX Extensions Validation and the ASP.NET 2.0 Validation controls is that the ASP.NET 2.0 AJAX Extensions validation registers expando attributes and script compatible with partial page rendering by using the script registration methods of the ScriptManager control.

When using ASP.NET 2.0 AJAX Extensions validators the control to validate and the validator should be inside the same UpdatePanel control.

Validators outside of UpdatePanel controls should use the script registration methods of the ClientScriptManager control.

Examples

This example uses two RequiredFieldValidator controls to validate user input. Error messages are displayed in a ValidationSummary control.

CS

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Validation Summary Example</title>
</head>
<body>
    <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    <asp:UpdatePanel runat="server">
    <ContentTemplate>
    <table cellpadding="10">
      <tr>
         <td>
    <table style="background-color:#eeeeee; padding:10">

               <tr>
                  <td colspan="3">
                     <b>Credit Card Information</b>
                  </td>
               </tr>
               <tr>
                  <td align="right">
                     Card Type:
                  </td>
                  <td>
                     <asp:RadioButtonList id="RadioButtonList1" 
                          RepeatLayout="Flow"
                          runat="server">

                        <asp:ListItem>MasterCard</asp:ListItem>
                        <asp:ListItem>Visa</asp:ListItem>

                     </asp:RadioButtonList>
                  </td>
                  <td align="center" rowspan="1">
                    <asp:RequiredFieldValidator
                          id="RequiredFieldValidator1"
                          ControlToValidate="RadioButtonList1"
                          ErrorMessage="Card Type."
                          Display="Static"
                          InitialValue="" 
                          Width="100%" 
                          Text="*"
                          runat="server"/>
                  </td>
               </tr>
               <tr>
                  <td align="right">
                     Card Number:
                  </td>
                  <td>
                     <asp:TextBox id="TextBox1" 
                          runat="server" />
                  </td>
                  <td>
                     <asp:RequiredFieldValidator
                          id="RequiredFieldValidator2"
                          ControlToValidate="TextBox1" 
                          ErrorMessage="Card Number. "
                          Display="Static"
                          Width="100%"
                          Text="*" 
                          runat="server"/>
                  </td>
               </tr>

               <tr>
                  <td></td>
                  <td>
                     <asp:Button id="Button1" 
                          Text="Validate" 
                          runat="server" />
                  </td>
                  <td></td>
               </tr>
            </table>
         </td>
         <td valign="top">
            <table cellpadding="20">
               <tr>
                  <td>
                     <asp:ValidationSummary id="valSum" 
                          DisplayMode="BulletList"
                          EnableClientScript="true"
                          HeaderText="You must enter a value in the following fields:"
                          runat="server"/>
                  </td>
               </tr>
            </table>
         </td>
      </tr>
    </table>
    </ContentTemplate>
    </asp:UpdatePanel>
    </form>
</body>
</html>