What do I replace CustomDomValidator with?

Hi,
I am migrating old .net 1.1 code which has the CustomDomValidator, which does not work on .NET 3.5. What do I replace with?
Thanks,


<cc1:CustomDomValidator id="custDVPage" runat="server" ClientValidationFunction="ValidatePageTextBox" Display="Dynamic" ErrorMessage="* Please specify a valid natural number greater than Zero"></cc1:CustomDomValidator>