XP machine loses connection with other desktops on LAN but not network printer or internet

I am setting up a small office network.  There are 2 vista desktops, 1 win7 desktop and 2 XP desktops and a network printer connected to the router.  There are 2 folders on one of the XP machines that need to be shared among all machines.  There is no server, it is a peer to peer network.  The 2 XP machines have static IP’s.  All others are dynamic.  The vista and win7 desktops have no problem accessing the folders, they work consistently.  The second XP machine will connect to the network and access the folders every time after a reboot but after 15 to 30 minutes it no longer sees the other machines on the network.  It does continue to access the internet and the network printer in the office.  If I ping the IP address of the host XP machine it does recognize the host XP as being there but it will not recognize it any other way until I reboot the machine (not the host).  I expect that it is a DNS problem but I don’t know what to do with it.  The XP desktop that loses connection is SP 3 and has Norton antivirus installed.  I have tried shutting down the firewall and disabling Norton but the problem continues.  I am thinking of deinstalling Norton but hate to do so because I’m not sure that is causing the problem.  I am going back to the customer location tomorrow.  I would appreciate any ideas to help get this resolved.