Minha melhor tentativa, boa sorte!
O cara em este site cita o artigo da KB 897103 " Você recebe uma mensagem de erro "Acesso negado" quando tenta acessar a pasta Documentos compartilhados no Windows XP ":
I seem to have solved the problem of all of the XP Home computers
(with SP3) on the network no longer being able to access the shares on
the network's one and only machine running XP Pro (with SP3) by
re-enabling Full Control permissions for the Shared Documents folder
to the Everyone account on the XP Pro machine.
Além disso, esse cara tem sua própria solução:
Solved. Turns out it wasn't just one XP Home workstation, it was all
XP Home workstations unable to connect to the server. Must of been
cached on other two that seem like it was working.
For other who run into this problem even though you know Win7 network
configuration is correct, check the network card configuration:
Control Panel -> System -> Device Manager -> Network Adapters, and
open the network card's properties. Under the Advanced tab, make sure
Flow Control and Interrupt Moderation is ENABLED! If these are
disabled, Win XP Home workstations will see the server, but will error
with permission error when trying to connect.