ASP.net with ajax - Check box control

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ASP.NET

Ajax

I m developing a web application for accounting. I need to load the values to combo box from DB when check box checked. But when Check box checked page should not reload. I tried several time with ajax update panel. But it was not success.
I dont know the correct way of using ajax update panel control.
Please guide me. Top urgent.

'Belove is the Check box event what i wan to call

Protected Sub ChkWHTBand_CheckedChanged(sender As Object, e As EventArgs) Handles ChkWHTBand.CheckedChanged
    GetTaxMaster()
End Sub

web page coding for specific event

<td class="auto-style48">
                               <asp:UpdatePanel ID="UpdatePanel2" runat="server">
                                   <ContentTemplate>
                                       <table class="auto-style42">
                                           <tr>
                                               <td>
                                                   <asp:CheckBox ID="ChkWHTBand" runat="server" Text="Is WHT Applicable" OnCheckedChanged="ChkWHTBand_CheckedChanged" />
                                               </td>
                                           </tr>
                                           <tr>
                                               <td>
                                                   <asp:DropDownList ID="ddTaxWHTPicker" runat="server" Font-Names="Tahoma" Height="22px" Width="150px" onclick="ddTaxWHTPicker_SelectedIndexChanged">
                                                   </asp:DropDownList>
                                               </td>
                                           </tr>
                                       </table>
                                   </ContentTemplate>
                               </asp:UpdatePanel>
                           </td>