My younger cousin has cable internet but his Mom and Dad opts for the cheaper modem so only one port direct into the PC. His Mom is on his @ss constantly biotching about how she can't use the internet when he is plugged into the modem.
So today out of darkness of heart, I went over and opened up the PC and dropped in a spare nic I had laying about my room. So this is the first step. You need 2 network cards inside the PC. They run cheap but so do switches and sh|t.
Next I grabbed his 25 foot LAN cable (Xbox to PC) and peeled back about 3 inches of the rubber coating. DON'T cut it just remove the outer layer. Inside is 8 wires only 4 need to be cut and spliced. They are orange, green, white/green, white/orange. (white stripes) Cross connect the solid orange to solid green and black tape. Cross connect white/orange to white/green and black tape then pat yourself on the back you just made a crossover cable. This also comes in handy if you want to hook up two 360's and have a LAN party!
Now here is the tricky part. On the PC figure out which nic is connected to Xbox by keeping the internet on and the xbox off. The card should have a BIG red X on it. Go ahead turn the Xbox on and if the X goes away then you have the correct card. Rename it XBox 360 or something. Next right click go to properties of the XBox nic.
Inside here look for (normally the 4th line) "Internet Protocol TCP/IP" select it and properties button. Select "use the following IP address"
Subnet Mask: 255.255.255.0
Leave the rest blank and click ok then close all that out.
Run cmd or command prompt. In the terminal window type ipconfig /all then look for the DNS Servers numbers on the internet. Write them down at most you need two.
Right click on the other nic (the internet) and Advanced or Sharing then select "Allow other network users to connect through this computer's Internet Connection". Done with the PC!
Now move to the Xbox. If you can't log into Live and it gives you the connection error; go to Settings, and then select System Settings. Next select Network Settings, Wired Network, and Configure Network.
On the Basic Settings tab, select IP Settings, Manual, and IP Address.
Enter 192.168.0.100, and Done.
Select Subnet Mask, enter 255.255.255.0, and Done.
Select Gateway, enter 192.168.0.1, and Done.
On the Basic Settings tab, select DNS Settings, Manual, and Primary DNS Server.
Enter the Numbers of your internet DNS you wrote down, and Done.
Then do the same for the Secondary with another DNS number and Done.
Select Done again.
Press B on your controller, and then select Test Xbox LIVE Connection.
The PC has to run all the time you are using the XBox but the good news is your Mom can't complain about being on Facebook anymore.
Last step charge your Aunt and Uncle $20 for the work you did.