You could set InitialValue = "Search..."
<asp:RequiredFieldValidator ID="rfvSearch" runat="server"
ControlToValidate="Search" ErrorMessage="please insert text"
ValidationGroup="Search" SetFocusOnError="True" Display="None"
InitialValue="Search...">
</asp:RequiredFieldValidator>
Doing this, error message will be shown unless Search
contains something different to Search...