XP and Vista sharing after replacement of crashed PC

I had a home network consisting of a wireless laptop and two desktops, all on XP SP2, connected via a Netgear DG834G wireless router connected to a broadband Internet connection. This network used to work fine with all machines having access to the Internet and to each other. Then, one of the desktops crashed with a motherboard failure and was removed from the network. From that moment on, although the laptop and remaining desktop had access to the Internet they could not share files. The defunct machine has been replaced by a new one running Vista and the problem continues with all machines having access to the Internet but none having access to either of the others.

I have tried to change the ICS settings but I get a message saying that I don’t have access to be able to change the ICS settings and “This setting can be changed in the Advanced Properties of the shared connection on the Internet Sharing computer.” I get this on both XP machines.

I have pinged the machines. They can all see the router on 192.168.0.1 and there is also a positive response from 192.168.0.4 which is an IP address that does not correspond to any of the current machines and I assume it is the IP address of the crashed machine which is no longer connected to the network. All the other pings failed.

I have checked all the cable connections and I have rebooted all the machines (several times) and the router.

On the Vista PC, Network discovery, File sharing and Public folder sharing are all On and Password protected sharing and Media sharing are Off. Internet security is provided by F-Secure and its firewall is enabled although when I disable it, it doesn’t seem to make any difference.

On the desktop XP machine Internet security is also provided by F-Secure and on the both XP’s firewall is On. F-Secure was installed after the third XP machine crashed and was removed from the network. I haven’t got round to installing F-Secure on the XP laptop. It is till using McAfee Antivirus but not a 3rd party firewall.

On the Vista machine in Network and Sharing Center the only entities that appear are the Netgear router and the Vista machine itself. After clicking on View Full Map, the correct map is drawn with the two desktops and the laptop hardwired to the router and the laptop wirelessly linked to the router. However, only the Vista machine is accessible; if I move the arrow over any of the other icons it remains an arrow rather than becoming a pointing hand.
Neither of the two XP machines registers anything other than themselves in Network Places.

I have tried “netsh winsock reset” but this does not give me access to the ICS settings.

I have changed the name of the workgroup on all three machines and this has made no difference.

I have used the “Create a new connection” and “Add a network place” and “Set up a home or small office network” wizards and they have had no effect.

My guess (and is no more than that) is that the problem has arisen because the crashed machine was the Internet Sharing Computer and there appears to be no way of disabling those settings now that that machine is not there.

I'd really welcome some help with this!