I have been having download speed problems and this is part of the answer I got from TalkTalk, I don't know if it helps you. They keep mentioning "modem" but they don't support them now.

Please ensure the modem/router is connected to a filter via an RJ11 cable (the Black 2 Metre RJ11 Cable we supplied). This RJ11 cable must connect into the DSL socket of a filter that is DIRECTLY connected into a BT-installed, wall-mounted telephone socket.

PLEASE NOTE: A telephone extension cable should not be used to connect the modem, as this was never designed to carry a broadband signal.

ALSO: Ensure that EVERY equipment attached to the phone line is connecting through a DSL filter (2 of these were supplied with your broadband modem). Unfiltered equipment (Such as Phones, Fax, Answer-Machine, Cordless Telephone Base Units, Alarm Systems, Surge Protectors, Sky Digital Set Top Boxes, Dialler Boxes, literally anything attached to the telephone line) can interfere the broadband reception of your modem. If you are in doubt as to whether a piece of equipment attached to the telephone line is causing a problem, simply disconnect all devices (except the modem)and try your broadband connection again.

Was it OK before you changed to a different firewall in which case its likely to be the new firewall settings. As Woolwell suggests try disabling the new firewall BUT before you do so disconnect from the Internet and make sure that the Windows firewall is up and running (it should kick in automatically) before you reconnect. Accessing the Internet without an active firewall is a big NO NO.

Woolwell. I have a cable connection and seperate router, Virgin refer to the box that connects to their cable as a modem although strictly speaking it isnt.

Some software firewalls have an option to block all network traffic. If this is set to block then you wont be able to connect to your router with your browser even with a cable connection.