There have been occasional reports whereby the removal of certain malware can cause the loss of all connections to the Internet.

Fortunately this event is fairly rare, but what the malware has done is to insert a piece of software called an LSP (Layered Service Provider) into the Windows TCP/IP handler.

It is known that using Adaware 6 to remove this type of malware on an XP machine, or manually deleting the LSP.dll can subsequently damage the TCP/IP protocol.

Because TCP/IP is built into Windows XP, it cannot be removed or re-installed (as you could in previous versions of Windows) and as such you would have to re-install XP to fix this problem.

Microsoft have since issued two articles which may help in these circumstances :-

KB299357 ~ How to reset Internet Protocol (TCP/IP) in Windows XP.
KB811259 ~ How to determine and recover from Winsock2 corruption.

A simpler solution may to to try either:-

LSP-Fix a free utility from cexx.org
WinSock XP Fix freeware by Option Explicit (which is also said to work in Windows 2000).

Quotes:-

"LSP-Fix repairs Winsock 2 settings, caused by buggy or improperly-removed Internet software, that result in loss of Internet access".

"WinSock XP Fix offers a last resort if your Internet connectivity has been corrupted due to invalid or removed registry entries."