After much troubleshooting and non-stop testing, we finally successfully played rounds of Unreal Tournament 2004, Call of Duty 2 (notorious for not being able to see games), Farcry, Warcraft 3, Aliens vs Predator 2, Command and Conquer 3, Supreme Commander, and more without anyone being dropped, and everyone being able to see the game instantly in the available games list. This was with a group of 12 people on a LAN.
One quick note, all these things I'm recommending to change are not for a permanent setup. These are settings for when you are having a LAN party.
First, the obvious things.
1. Turn off Windows Firewall. I know people are paranoid about this, but trust me. If you want to eliminate every possible hindrance to gaming, just turn it off.
2. Turn off all other antivirus / firewall utilities. These include Norton, Mccaffe, AVG, Zone Alarm, Avast, etc... Whether or not the utility has a scanning function or firewall is irrelevant. Just turn off all these types of programs regardless of if it makes sense. If computers made sense, you wouldn't be reading this. For Norton, please PLEASE make sure you have the Internet Worm Protection DISABLED before turning it off. If you disable Norton but did NOT uncheck the check box for worm protection, that setting is still enabled, whether Norton is running or not. Trust me. Also, worm protection is NOT the same as antivirus. There is a checkbox that specifically says "enable norton internet worm protection". They like to hide that one within the menus, so go find it. If you are really worried about being hacked while gaming, unplug the internet cable going into your router for the time being.
3. Ok, it's all turned off right? WRONG. If you look in Windows Services (go to start > run > type in services.msc and press enter), you will probably see entries for the programs you have just disabled, and guess what? Most likely they say "started"! To fix this, just right-click, go to properties for EACH ENTRY, and click "stop". Once it's stopped, also change it from "automatic" or "manual" to "disabled". Remember, these settings we are changing you will probably want to change back when you are done with your LAN party.
4.Setup static IP addresses - again, this shouldn't matter, but if computer networking worked / games were coded intelligently, you wouldn't be reading this.
5.Join the same Windows workgroup. Sound stupid? Fine don't believe me, but when your game isn't visible on the LAN, don't cry to me about it.
6.Open up your Network Connections under the Control Panel. No not 'My Network Places', NETWORK CONNECTIONS. Click on 'Advanced' at the top, then go to 'Advanced Settings'. On the Adapters and Bindings tab, you will see a section labeled 'Connections'. This is your 'network stack order'. Figure out what in that list is your actual Ethernet adapter, and move it to the TOP of the list using the arrows on the right.
7.Ok, now for the real kicker. This is the one that fixed 99% of our problems. Go back into your network connections. Aside from '1394' and 'Internet Connection', the only adapter in here that should be enabled is the one that is physically connected to your network. Hamachi, VPN, VMWare, a second network port on your motherboard, a second ad-in PCI card network card, wireless, DISABLE THEM ALL.
You may be thinking "ok, now I know for a fact that this is BS." I'm afraid you are wrong. My friends and I have extensively tested this with multiple people hosting and trying to join. If you are having problems, at least give it a shot instead of just shooting it down. Trust me, it works.
8. I've neglected to mention this part, but obviously check for newer drivers for your network card, and also that the cables you are using are plugged in and are in good physical condition. Also ensure your router / switch isn't too ancient. If you are trying to figure out Uplink port configurations and crossover cables, I suggest just buying a new router / switch that auto detects that stuff. For a switch, I suggest the D-Link DGS-2208. Its an 8 port gigabit switch. $50 normally, I purchased two for $30ea (after rebate. (Please dont start ranting about how you hate dlink or something. It is a suggestion).
9.For most other problems you will have to configure network settings in the game you are running. Do yourself a favor and DISABLE PUNKBUSTER wherever possible. In CoD2, you can configure ISDN or LAN, etc... for CnC3 you can configure what IP is used for Internet/LAN games... you will have to search around for help on the specific game you are trying to play.