12 networked computers don't share files or printer

HARDWARE....
I have 12 computers (exactly the same) running WindowsXP (newly
installed). I have 2 hubs connected to 6 computers for each hub. The
hubs are uplinked together. The network is wired correctly since they
all can access the internet over a DSL/router and Modem which is
connected to one of the hubs. There are 2 USB printers connected to 2
computers which print a perfect test page from each computer. The
printers are setup to be shared and show up as a shared printer in the
"network printer" (cannot think of the correct area) control panel.

PROBLEM...
When I try to use the wizard to setup access to a shared printer on
the other 10 networked computers, no printers show up in the "select a
network printer" box after the system searches the for a networked
printer. I have tried to enter the computer name and printer name
with the same result. Also, I have found that none of the computers
can share files. It seems as though the network settings are not
correct. I have been through the network setup many times without any
luck. Can anyone help? Thanks.