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I'd be surprised if this is an ntl boradband issue, more something to do with your own computer setup.
Have a look at your Win XP security centre settings (check in control panel). Is the windowws firewall on. If so disable it and see if there is any difference.
I assume you are connecting direct to your modem / STB (no router). USB or ethernet. Check your ethernet drivers are current, windows is fully patched (windows update), etc.