TCP/IP home networking tutorial

Thanks! That was a big help. You completely solved my home networking problem which I've been researching for days. Finally the answer.

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Dear friends,

I have a small office lan (4 computers with win2000 professional. User1, user2, user3, user4).
Recently I installed a DSL line at user1 and I wanted to share net at the other computers. The first three days was a disaster. User 1 & 2 had disappeared from the network while at their sharing session there were missing ipc$ and admin$ and messages like ‘server is not accessible’ appeared when trying to connect to them.

Looking at the Internet I realized that it was a virus matter. So I downloaded Panda antivirus (I had Norton but it hadn’t found anything) and found some viruses and Trojan horses. The connectivity problems solved. The next step was to adjust IP addresses to TCP/IP of the local network.
User1: 192.168.0.1
User2: 192.168.0.2
User3: 192.168.0.3
User4: 192.168.0.4
Subnet mask :255.255.255.0
DNS server and gateway:192.168.0.1

It worked for 3 days. I had internet sharing!

Problems reappeared when I installed emule at user3. Suddenly Internet Connection Sharing was lost and user 1, 2, 3 cannot ping each other. (I imagine that it is something with proxy settings but I am not familiar to that)!

Please help, I am desperate
Costas