Setting up LAN in exisiting Wireless Network?

I am trying to set up a LAN over my exisiting wireless
network. I have 3 computers each running XP. One
computer does not have wireless, however it is connected
to the wireless router through an ethernet cable. This
computer does not recognize any other computer on the
network. Also I have a laptop running XP that is
wirelessly connected but after setting up LAN (same
workgroup diff comp name) it does not recognize anything
on the network. On both these computers when I try to
look at computers on this workgroup, it tells me I may
need to contact sys administrator. However, I am the sys
administrator and there should be no reason to approve
any computer. On my third computer, I am able to see the
first computer mentioned and able to use the printer on
it and get files from it. But I do not see the laptop on
the network.

I have double checked to make sure workgroup names are
the same and the comp names are different. The internect
connection on all the computers is still ok. However
only one comp recognizes another comp but not both. The
other two comp do not recognize anything on the network.
I guess the center of my network is the UPnP device
(wireless router).

Can anyone help? Thanks