Pentesting Network Share Access via wireless

Hi All,
     While doing a pen-test on a wireless network, I noticed that the router
was not configured properly, and was giving out IP addresses to everyone.
Now after noticing all the host pc's on the network with nmap -sP, my
question is if I don't know the passwords for any of the host what is the
best way to do a dictionary attempt on the guest, or administrator accounts.
All the internal host are windows XP.

Thanks in advance, all help is welcomed.


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