Diagnostic tool only says "contact manufacturer"
Which tool? If you mean the XP Diagnose Network Problems (e.g. via IE's Tools menu) it would say more than that. Another way to go on that OS would be netsh diag gui (e.g. from a cmd window or via Run...)
The kinds of reasons that I can think of why one account would not connect while the hardware and the OS were otherwise available and willing would be proxy settings. That XP Diagnostic fails miserably at informing you about errors in that regard.
Can the problem user connect using something other than IE? E.g. another browser, or XP's Help and Support (Win-F1)? In any case I think you would get more informed help about connectivity problems from an XP forum which specialized in Networking.
Good luck
Robert Aldwinckle
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