I could guess ..... perhaps if you also pose your question also in Italian it would make it easier to respond. Here's my understanding:
You have 2 computers and a router that connects to a DSL connection.
You recently installed an ethernet card / network interface card (NIC) into one of the computers. You want to communicate between the computers via ethernet. You also want each computer to connect to the internet via the router.
When you connect the new computer to the router, the light on the new ethernet card flashes. When you connect the two computers together the light on the new ethernet card flashes rapidly.
It appears that you likely have hardware problems - with the blinking lights.1) If you are connecting the computers via the router, make sure that the cables are straight-through cables and neither of them is a crossover cable. (Unless the router provides automatic MDI/MDI-X autosensing).
2) If you are testing the peer-to-peer connection without the router, make sure that the cable is a crossover cable.
How to tell a straight vs. a crossover cable? Hold the two connectors so that they are pointing in the same direction. Have the flat / transparent side of the connector up - so you can see into the connector easily. A straight cable will have the wires arranged the same on both connectors. A crossover cable will have the wires arranged differently - from one connector to the other. See: