Outlook Express has a nasty habit of screwing up its settings.
Go to Tools > Accounts.
Click on the 'Mail' tab.
If you've only got one email account it will probably already be highlighted. Otherwise click on the account name to highlight it.
Click 'Properties'.
Select the 'Servers' tab.
Check that the incoming (POP3) server address is 'mail.talktalk.net'
Check that the outgoing (SMTO) server address is smtp.talktalk.net
Check that your username is shown in the form '
[email protected]'.
Delete your (asterisked) password and retype it.
Check that there's a tick alongside 'Remember password'
Check that there are no ticks anywhere else.
Click 'Apply', 'OK' and 'Close'.
If that doesn't work, start as before but (instead of selecting 'Properties') select 'Remove'. When asked to confirm this action, do so.
Then click 'New > Mail' and create the account again by following the on-screen instructions (using the POP3 and SMTP server addresses from above).
Chris