I have a huge dilemma. I know something about networking but I have never seen something like this. I will write down what was, is and I already tried.

First what was: I have a PC with 2 network cards, one onboard and one on PCI. On the PCi network card the internet cable is plugged in. The onboard network card was connected by a CAT 5 LAN cable to another PC in my house. I installed a proxy on both f the computers, made a LAN and shared the internet through the proxy. Everything worked fine. Then one day, the LAN just stopped working.

What is: There is absolutely no connections between the 2 PC's. Ping doesn't work (Request timed out) and in the Local Area connection status packets are being sent by both PC's but none of them receives any. So on both systems it says Local Area Connection is now connected, Packets sent XXX (Number rises) Packets received 0.

What I already tried. First I thought it had to do something with windows. I reinstalled windows on both PC's, same result. I changed the network cable with one that is surely good (Has been tested) same results. I bought a laptop, has vista on it, tried connecting to both PC's one at a time, same results. Bought a new network card, connected through that, same results. A friend came over, he brought his wireless router, I have connected with my laptop through wireless and with cable with one of the PC's. Internet runs just fine on both, no connection between the 2 PC's. Tried with a switch, same results, tried with a direct cable between 2 PC's same results. Renewed the IP address, same results. Used manual IP: on one PC on the other, Subnet mask, default gateway none (since it was a direct connection). DNS server none.

What else can I try? I used ipconfig /release, restarted the PC, repair the connection, still no packets received by any of the PC's, but if I plug in the internet in the same network cards and put everything on automatic (Obtain IP and DNS server automatically) it works. So I don't think that the network card has a problem. Changed cables a lots of times with direct, switch and even with router connecting, packets still not received. I have no idea what to do next, I tried everything I could think of. Any ideas?