Ok, firstly

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Secondly do you have a network port in the back of your PC?
If you do, it might be worth trying to get that set up for you rather than having you continue with the USB connection. The NTL software supplied on the disc is only really there to allow you to connect via USB, and it's a bit flakey (as you've already discovered).
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Before we try that though.....
Turn everything off.
Turn the modem on and let the lights settle, turn your PC on and let the lights settle.
Is it working now?
If not, do you have a firewall installed that could be causing problems?