Unabel to access clean PC

Strange problenm with a computer, maybe someone has an idea that will work.

The client was informed that they were about to be denied internet access by the ISP due to virus proliferation (they were broadcasting viruses from the network). The PC was running Norton, which was removed, and replaced by Mcafee V 7.1, and updated. The PC was returned to the client, tested, and works 100%. The probelm is that this PC acted as the gateway for the rest of the computers to access the internet, through it's second network card. All the settings are correct, and itnernet works fine on the gateway PC (the one that was infected). It can also see and access all the other computers on the network. The probelm is that the reverse is not true. None of the other computers are able to see or access the gateway computer, or access the internet. However, they are able to ping both the gateway PC and the remote gateway at the ISP. They are even able to open a telnet session on the ISPs router (not the best of security, I realise! ). I tried modifiying the gateway address on the other PCs to point to the other NIC on the gateway PC, and I've also activated the guest account on the gateway PC.
None of this has made any difference for accessing the itnernet. However, clients are able to download their e-mail from the gateway PC, which downloads and splits the e-mails from the ISP for the various user accounts. I was thinking of possibly re-installing the NIC drivers, or possibly all of XP itself. I would like to avoid such drastic actions if possible however, so your alternative ideas would be most welcome.

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Strange that users can connect to the ISP router! As I am not sure what the problem is, I'd suggest going back to basics.

With the gateway PC set up as it is, it is effectively acting as router. For such a system to work there needs to be three seperate networks:

1. The internet, beyond the ISP router
2. The network between the gateway PC and the ISP router
3. The internal network, connected to the second NIC on the gateway PC

Each network must have a unique subnet (part of the IP address covered by the 255s in the subnet mask).

For computers connected directly to network 2, the default gateway is the ISP router's interface on that network.

For computers on the internal network (3) the default gateway is the gateway PC's interface on network 3. Always think of a gateway as the path out of the network. It must be part of your network or you won't be able to get to it. Therefore setting the gateway for network 3 as the gateway PCs external IP (which is on network 2) won't work.

One arrangement that may end up with symptoms as you describe them is if the Gateway PC is acting as a bridge rather than a router. That is if you have the same network subnet settings for networks 2 and 3.

This is an arrangement that should work:
Network 2: 192.168.0.0/255.255.255.0; Gateway internal IP of ISP router (e.g. 192.168.0.1)
Network 3: 10.0.0.0/255.255.0.0; Gateway 2nd NIC ip address on internal network. (e.g. 10.0.0.1)

This arrangement will not work:
Network 2: 192.168.0.0/255.255.255.0; Gateway internal IP of ISP router (e.g. 192.168.0.1)
Network 3: 192.168.0.0/255.255.255.0; Gateway 2nd NIC ip address on internal network. (e.g. 192.168.0.200)

To test the system try a TRACERT WWW.GOOGLE.COM. Do you get to the google IP address? If not where does it get stuck.