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Windows 2000 ...
Microsoft KB # 318584 says to back up your registry and then do something like this ...

back up and then delete these two registry keys:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Winsock
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Winsock2

Remove TCP/IP from each one of your network configurations and reboot. Reinstall TCP/IP and test

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Windows XP ...
won't allow you to remove and reinstall TCP/IP .. but Microsoft document 299357 might help you reset the IP information in the registry

netsh int ip reset resetlog.txt

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if this doesn't work, try LSP fix or SpyBot