Requirements:
Windows Computer(XP,Vista,Se7en)
Ethernet Cable(xbox comes with one)
Your laptop most be connected to the internet(preferably via wireless)
Ok the first step is to change the settings on the computer, so:
[Windows 7]
Locate the Network connection (Its a bunch of bars for wireless connections) in the tray, if it isn't there look for it on the all icons menu. Click it once to bring up a dialog type thing, Click the Open Network and sharing center link.
Click on the name of your network towards the right.
{Example of clicking the network name on the right}
Now click the properties button with the (ugly) blue and yellow shield, if you are an admin account (default) it will notice this is a trivial task and not show a UAC dialog so don't worry if you expected it to appear.
Go to the sharing tab and check both boxes, and click ok until the dialogs are closed, then close network and sharing center.
Plug one end of the Ethernet cord into your computer, and plug the other into the back of the Xbox360.
Turn on the Xbox, go to system settings and go to network. make sure the settings are set to automatic, and choose the test pc connection option. The name of your computer should show up. Choose it.
Then choose the test live connection and it should work.
From now on to use internet on xbox simply plug your computer into it via the Ethernet and it will work automatically.
[Windows Vista]
Click Start->Control Panel->View network status and tasks OR Network and sharing center depending on view mode.
Click on View Status, then click the properties button (the bottommost properties button, with the shield.
Accept the UAC Prompt.
Go to the sharing tab.
Check BOTH boxes on the sharing tab and click ok until the dialogs are gone. then close the control panel window.
Plug one end of the Ethernet cord into your computer, and plug the other into the back of the Xbox360.
Turn on the Xbox, go to system settings and go to network. make sure the settings are set to automatic, and choose the test pc connection option. The name of your computer should show up. Choose it.
Then choose the test live connection and it should work.
From now on to use internet on xbox simply plug your computer into it via the Ethernet and it will work automatically.
[Windows XP]
Open Control Panel
Click Network Connections
Choose your connection from the ones in the window
Right click and click properties.
Go to the sharing tab.
Check BOTH boxes on the sharing tab and click ok until the sharing dialog is gone, leave the rest open.
Turn on the Xbox, go to system settings and go to network. make sure the settings are set to automatic. Choose to test connection to live, wheter or not it succeeds open the sharing window back up and click settings, the Xbox should have added 1-2 entiries to this list (they are labeled xbox)
If they are checked then go ahead and connect to live, if not check them and accept the dialog and connect to live.
If a live feature ever fails return to this dialog and see if it added a new entry you need to check.
If you need any help with any of this post and I will help you.