Validation question. At least one of the five fields must be completed.

I'm working on a telephone directory for my company's intranet and i am trying to create an advanced search function with text boxes for Forename, Surname, Extension, Mobile and Department. I want a validator control to avoid users trying to search on five blank fields. Therefore, at least one of the fields must have something in it before it will fire.  Does anyone know how I can do one of these? My asp.net code is below:
      <asp:Panel runat="server" ID="pnlAdvancedSearch" Visible="false">
            <table>
            <tr><td>Forename: </td><td><asp:TextBox ID="txtAdvForename" runat="server" /></td></tr>
            <tr><td>Surname: &l ...