Hi rhainman,
I have found the info below on NTL's Cable Modem Troubleshooting Tips which may help
Windows XP built-in firewall
Some users report difficulties getting Windows XP to work when first connected. It is possible that the built-in firewall might be causing problems with the strange requirements of the cable modem DHCP system. It is suggested that you should not enable the built-in firewall until after (a) you have a working cable connection, and (b) you understand how to configure the built-in firewall. You should only consider enabling the firewall if your XP PC is directly connected to the cable modem. You should disable the firewall if there is a NAT router and/or home LAN between the XP PC and the cable modem.
Open Control Panel.
Double-click Network Connections.
Right-click the appropriate connection icon, and select Properties.
Click on tab Advanced to bring it to the front.
To enable the firewall, check Protect my computer (etc); or
To disable the firewall, uncheck Protect my computer (etc).
Click on button Settings...
With tab Services at the front:
check any services offered on your PC that you wish outside users to be able to connect to. If there are no such services, uncheck all boxes.
Click tab ICMP to bring it to the front:
check Allow incoming echo request.
check all the Allow outgoing options.
Click OK to exit Advanced Settings.
Click OK to exit Connection Properties.
I have also attached a Step By Step Guide from ntlworld regarding how to configure your existing ntl Broadband connection for a new PC.
Which may offer some further answers..
