Have you right-clicked on My Computer and selected 'Sharing' for each machine? You need to make sure that each machine gives the others permission to view contents.
Further, double check that each machine is uniquely named on the network, and that the group setting is IDENTICAL on each machine.
Couple of pointers to start.
Be safe.
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'I'll take control...'
'You have control.'
'Well give me it then!!'