Try reinstalling TCP/IP by following these instructions EXACTLY:
To reinstall TCP/IP in Windows XP, right click the network connection and select Properties. Click the "Install" button, select Protocol and click the "Add" button. In the next box, click the "Have Disk" button and, in the line "Copy manufacturer's files from:", type C:\Windows\inf and click OK. In the next box, click Microsoft in the left pane, then Internet Protocol (TCP/IP) in the right pane and click "OK". Close all open windows and reboot the PC.