There is a very useful program, in BackTrack you will find it pre-installed, named rpcclient. This tool executes client side MS-RPC functions and is part of samba. rpcclient manpage
Open up a terminal and execute:
rpcclient -U "" target_ip_address
In password prompt just hit enter.
If the connection is successful you should see a rpcclient prompt.
to dump a list of user accounts.