problem between 2 computers on home network

I have a wired home network with three XP desktops and one Vista
laptop. One of the desktops is the main one. I can't get the main
one and the laptop to see each other properly. If I go to Network it
is there. If I click on the computer it brings up its folders. OK so
far. But with this pair of computers if I try to open a folder it
says ".... not accessible.. Might not have permission.... " and then
"Not enough server storage to process this command".

Both drives have share on. I tried temporarily turning off the
firewall, that didn't help. The Windows diagnostics doesn't find a
problem with the network. All other combinations can see each other.

How can I try to fix this problem?
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