In Settings -> Network Connections you should find "Local Area Connection". Open it, click on Properties, scroll down to Internet Protocol (TCP/IP), click on that, click on Properties, and check "Obtain an IP address automatically" and "Obtain DNS server address automatically". Click all the OK buttons to get out of these windows.
From previous replies to your question, I am not sure if this will help, but here goes:
In Win XP you can "turn off" the ethernet port, in a manner of speaking, by going to control panel>Network connections>right click on local area connection icon and click on "disable" in the menu. This will prevent the adapter from passing traffic in and out. To re-enable the adapter, use the same steps, only choose the word "enable" from the menu.
You can always tell the status of your adapter by right clicking on the local area connection icon. If "disable" is the first word on the menu, it means the adapter is already enabled, and vice versa. Hope this will help you. Good luck.