Windows 7 does not include a standard Mail client (see http://news.cnet.com/8301-13860_3-10048142-56.html ). Windows Live Mail is for their @live and @hotmail addresses, which is why you can't register your @ntl address.

So you'll have to get Outlook for W7 if you want to continue to use a mail client on your PC

If it was me, I'd get a Googlemail account, and have all your ntl emails forwarded to it. You can set it up so that when replying in googlemail, it uses your ntl address as the sent from address too