C$ window pops up while logging on???

Problem:
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On my W2k network, there is one NT4 workstation client on which I’m facing this problem.

As the user logs on to this PC a window pops up

Incorrect username or password for \\computername\C$

Connect as: ______________
Password: ______________

Elucidation:
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This window also appears in accessing some applications. The user is ordinary user not with admin rights. But when I logon to that PC using Administrator accounts this windows doesn’t appear.
My Question:
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What should I do to solve this problem so that user don’t see that window when logging on & using some applications?