Help on troubleshooting ICS Host

I have turned ICS on but the clients on the network are getting no response
when trying to access the internet through IE.

So far I have carried out all the client troubleshooting (found in
knowledgebase) and they are obtaining a correct set of IP information (i.e.
dns server, gateway, IP address, subnet and dns extension). No connection is
occuring at either the name or IP address levels.

I have also set the Host to allow the clients to alter the connection and
also to dial when a request is made. No attempt to dial by the host is made
when a client attempts to access the internet.

On the Client, the internet connection icon is showing not connected". when
double clicking on it, it breifily says connecting, then connected. then
reverts back to not connected.

The Host is connecting to the ISP ok.

all machines are XP Pro SP1.

IF anyone can point me in the direction of a good troubleshooting guide for
ICS on the host, or has an idea on what could be wrong, I would be very
appreciative.