In Outlook Express
Click Tools> Options >Mail >New Account.
Fill in the NTL details as required by the 'wizard' and you will then have a new account listed when you click on Send/Receive.
You need to know the pop and smtp details, these are available from the NTLWorld site under 'Help and Support'
You cannot send emails by your SKY account as if it was from NTL but you can set OE to check other ISPs.
I have an account with Madasafish and still have my Windows Mail (Vista) checking this with my 3 Sky accounts.