PC cannot see most of the network (and vice versa)

I am running a network with eleven PCs, most with XP, some with XP
professional.
As the network has now exceeded the XP Pro limit of ten, I have acquired a
server with Windows Small Business Server 2003 installed on it. There is a
separate router, but I use the Server as my DNS Server.
Following this change, two of my PCs (both XP) cannot see most of the PCs on
the network from within Windows Explorer. They can see them all through the
command prompt and they have had their IP addresses allocated by the server.

Over the same weekend, I loaded Norton Internet Security 2009 on some of the
PCS including the problem ones. I have Norton Internet Security 2008 on all
other machines.
Norton Network map can see all the other PCs.

Please can anyone offer any suggestions?

Thanks

Rob