I had a bit of trouble last week, when I couldn't get through to anything, because they were doing some maintenance work. As well as their helpline, they have a Service Status line which just guves a run down of various "service outrages" because of repairs to the system and so forth.

But if you can get through here, but not to other sites I'd be surprised if it's NTL to blame - normally it seems to be all or nothing.

I'd advise giving them a ring and asking advice.   

If it's not NTL, it might be worth trying using a different browser, if you've got one on your PC, and seeing if the problem crops up there too. If it doesn't, that would suggest you should try reinstalling your prefered browser.