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Can you confirm which type of STB you have - Pace or Samsung. Some of the older STBs used a crossover cable. To avoid issues here, use the cable you had between the STB and PC (the setup before you added the router) between the STB and WAN port of the router. Use new ethernet cables between the router and PC.
Make sure your PC doesn't have any retained settings in it if you previously used an ntl setup disk. Particularly in Internet Explorer (IE), check that the connections are all set to autodetect, and there is no preset proxy (use the Tools > Options menu).