HTTP Proxy authentication

We're running SBS2000 and one or two client computers occasionally, say
about once per week but not exactly that schedule, gets a box popping up on
the desktop stating "Your HTTP proxy requires user authentication, please
enter username and password" followed by three boxes for entering
information:
Proxy address: SBSSERVER:8080
Username:
Password:

In the top of the box in the blue bar is written "Please enter password
for..." but the rest is cut off and the box can't be expanded to read the
whole line.

Does anyone know what this is about. Our system seems to be generally
running well.


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