I have experienced this once and it also affected only one client.
The solution i used was to deinstall the Winsock 2 update, reboot the machine, delete everything like network adapter, protocol and client (network properties) and after reboot reinstall those items (override newer files with older once).

It not a fancy way, but give it a try. After this action, some applications will not work properly (like IE5), but after reinstalling them the computer logged on correctly and everything works fine.

And offcourse reinstalling the client is another option

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G.J. Minkels
Océ-Nederland B.V.
The Netherlands