I just posted this on my site and thought it might be usefull here 
Firstly, you will need
ALL of the following:-
- Internet connection (dial-up / ADSL / Cable Modem)
- Spare network card in PC with internet connection
- Spare network card in machine to share internet with
- EITHER a Cat5 Crossover lead OR 2 Cat5 Patch leads and a hub
- Some spare time!
If you are just sharing to one device, you will save money just getting a patch lead.
If you want to connect more than one device, you will need a hub. (You could use a router / switch, but for ease I will refer to hub for this walkthrough).
For ease, I will refer to the machine with the internet connection as PC and the device to connect to it as CLIENT.
The client may be a PC, Laptop, X-Box, PS/2 etc.
Follow this guide one step at a time. If you connect cables too soon it will be a lot more difficult!
STEP 1
Switch on the PC and make sure the internet connection is working.
Click on the Start button and
RIGHT click on "My Network Places" and select Properties.
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You should then see a screen similar to this:-
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It doesn't matter if yours looks a little different, as long as one is connected (your internet connection) and the other shows the no-cable icon.
Right click on your internet connection and select Rename.
Rename it to "Internet".
Right click on the other connection and Rename it "LAN".
Shut down the PC.
STEP 2 - Using CROSSOVER CABLE (scroll down for hub)
Connect the crossover cable to the PC and the Client.
Switch on the PC.
STEP 2 - Using a HUB
Connect the PC to the hub using a patch cable.
Connect the Client to the hub using another patch cable.
Make sure the hub is switched on.
STEP 3
Once the PC is running, check the internet connection is working!
Click on the Start button and right click "My Network Places" and select Properties (as before).
You should now see something like this:-
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Don't worry if your 'LAN' connection says it's unplugged, that is only because the Client is not switched on.
Right click on 'Internet' and select Properties.
Click on the Advanced tab at the top.
Put a tick in the box for Share Internet Connection.
You will see a warning telling you that an IP address will be changed to 192.168.0.1.
Just agree and continue.
Once this step is complete, shut down the PC.
STEP 4
Setting up the client is relatively simple.
You will need to configure IP information on the client to use DHCP.
If you have an X-Box - PS/2 console, just set your network settings to DHCP.
If you have a Windows 98/ME/NT/XP/2000 machine, you need do nothing at all!
STEP 5
Power up the PC.
Once up, make sure the internet connection works.
Power up Client machine.
DONE!