Sorry this response is so late. Are you still having this problem?
Have you using DMZ on your router? It's a setting where you can set one PC on the LAN to act as if it was connected directly to the modem. This does mean you lose out on the security that the router provides, but if your PC has adequate protection already then this shouldn't be too much of a problem.
If that doesn't work, the next thing I'd try is wiping your hard drive and starting a fresh.
Am I right in thinking that the internet works fine otherwise?