The modems that are provided free by ISPs are only capable of connecting one pc to the internet. If you want to connect a second pc there are a number of different ways, but if the 2 pcs are close together, the easiest way is to obtain an adsl modem router with 4 ethernet ports.
This device combines the adsl modem, which connects your system with the internet, and a router which connects the modem to your pcs and your pcs to each other.
These devices are not expensive, e.g. see here click here and you will also need 2 ethernet cables to connect the router to each pc.
You will have to put the Tiscali connection username and password into the router, which then connects to the internet. When you switch on each pc it will be able to connect to the internet straight away.
Hope this helps.