Have you tried re-making the TalkTalk email account in W7? Creating new one and/or re-entering all details. It could be as simple as a typing error in your user name. Look in the account details.
Can you access the Webmail when using XP? if so then there is no reason why you can't when using W7 unless it is your pc's security settings. You should be able to login to Webmail from any pc.