Firstly Welcome to the forums.
I hope you like it here.
I am sure a person who works for NTL will come accross your post in due time and can check your account for you.
You could try phoning NTL and asking for them to resend the firmware to the modem and see if it makes a difference.
Your computer, unless it has any bandwidth limiting programs or adware/spyware, should not play a part in the problem.
NTL will be able to run a speed check on your connection and make sure your modem is set up for the correct speed.