I have some problems with the LAN between my computer and that of my brother, and I thought maybe some fellow GOGer can give me some helpful advice on the matter.
Network adapter-wise, our computers look like this: we both have an on-board nVidia card, and a PCI Realtek one. The LAN goes through our nVidia cards; the ADSL cable is plugged into my Realtek card, and is shared with my brother.
Now, to the problem itself. There is this LAN connection between the two computers, which works well regarding file, printer and internet sharing, but seems not to exist when we try to play most any game on it. With most of the games we tried, the game simply doesn't see the game created by the other player, no matter which one of us is the host. With other games (for example, Sacred Gold), the client sees the host's game but cannot connect and it ends with a time-out. With yet others (for example, Emire Earth Gold), my brother can host a game, which I can join, but it doesn't work the other way around.
Here are the things I recall I've tried doing.
-Use some other combination of the network cards to establish the LAN.
-Update drivers (not that they are that new, anyway...).
-Disable antivirus/firewall (on both computers).
It really bugs me that we can't get a good LAN game, even though the connection itself seems to work all right. Anyone got any suggestions for me?

While I agree in general, I think that's not strictly necessary where there are two computers and one router involved... also I don't think you could even *find* a genuine hub for sale these days. If he does find one, it should at least be a cheap solution.
I'd also like to throw in my support for the "uninstall hamachi" suggestions previously.
Computer #1 has two IP addresses that will be recognised by computer #2:
* 192.168.0.1 (nVidia)
* 5.137.144.4 (Hamachi)
Computer #2 has two IP addresses that will be recognised by computer #1:
* 192.168.0.197 (nVidia)
* 5.137.154.20 (Hamachi)
When you try to join your brother's game, it might be trying to connect using the Hamachi adapter, which will use their drivers and potentially cause all sorts of connection problems.
Uninstalling Hamachi on both machines will leave only one shared IP address (192.168.0.xxx) and might make the games work again.
If this doesn't work, your best bet will probably be the router/ICS suggestions of the previous posters.

It seems uninstalling Hamachi has indeed solved the problem. We've tried playing Din's Curse (which didn't work before) and Total Annihilation, and they both worked. Funny, once we installed it because we couldn't get our games working... but at that time, we didn't disable the AV softrware monitoring the network, and that may have been causing the problem then.
Thank you, everyone. I'll mark HampsterStyle's first post as a solution because the removal of Hamachi was first mentioned there, but I'll give +rep to everyone else who gave me advice, and I'll ask my brother to do so, too.