If you have any Tiscali connection software remaining in the Laptop, uninstall it.
Then delete the Tiscali wireless "profile" - go into "Control Panel > Network and Sharing Center" and click the link at the left to "Manage Wireless Networks". In the widow that opens, highlight the entry for the Tiscali wireless connection and click the "Remove" button.
Now see if you can connect to your TalkTalk router. If it still doesn't bring up the 'connecting' dialog and the prompt to enter the encryption key, try setting it up manually. Still in the "Manage Wireless Networks" window, click the Add button and then select the option to "Manually create a network profile".