Do the PCs have exactly the same setup? E.g. you take the network capble, connect it to the PC and it works, - connect it to the Mac and it doesn't?

In that case you may try to find out whether your router is correctly set as your gateway in your mac configuration. I assume you are using a proxy for surfing the WWW? If you cannot surf the WWW without the proxy, you probably did not set your gateway. If you can, however, I would assume that your local software firewall on your Mac is badly configured.