Hi,
If you are with TalkTalk are you using their router.
http://is03.thegumtree.com/image/big/129865391.jpg
It could look like this .......but there are many different options.
The yellow cable you refer to is an ethernet cable ( I think )
http://www.scadametrics.com/images/Patch_Yellow_03.jpg
Your router will also need a Filter before the phone socket
http://ecx.images-amazon.com/images/I/31zSQGW9bBL._SL500_AA300_.jpg
Every phone / answerphone / Sky TV etc that is on the phone network will need a filter too.
If the router has been set-up downstairs and computers can connect to the internet OK then there should not be a problem with a wired connection.
Can you connect to the internet using the router - even from downstairs ?