Networking problem

I do small office networking, and I have a client who is
running 4 desktops with xp pro, and two with millenium.
They also have a windows 2000 server. As of now their
network is only peer-to-peer with the intention of
upgrading all pc's and to create a domain. Anyways, two
computers are unable to see the server, however they can
see the other computers in the workgroup. When i try to
map the network drive it tells me the network name is
already in use or logged on. That is not the case.
Finally, I changed the computer name, and I was able to
connect. Now the problem is back and changing the name
did not work. Any suggestions would be greatly
appreciated. Thanks