Win 2k NLBS

I have two Web Servers on Win 2k which are configured to accept requests on port 80 (http) as well as port 443 (https), and have installed NLBS on both these servers.
How do I configure NLBS so that it fails over when either of the Servers Web/HTTP service fails or stops ?
Because when I stop the HTTP services and try to telnet the server on either port 80 or 443, it isn’t accessible.
What configuration changes do I have to implement so that NLBS initiates a fail over when the requisite ports aren’t available on either one of the nodes in the cluster ?