Ethernet connection problem

Have small lan in house, up to 4-6 pcs at any one time. Two running Vista
and rest running XP, one Home and couple Pro. All legal, all updated to
latest "auto update" offers.

Two of the pc's can see each other, can see shares but will not connect
otherwise with "Workgroup" window usually hanging after double clicking in
Explorer to access the share. This happens going either direction between
the two pcs.

As I write this there's 2 XP Pro's, 1 XP Home and 2 Vista's turned on right
now. All have expected DHCP ip's from router and all can connect to
Internet. The two pc's in question can see each other but cannot connect to
the shared "Temp" folder on each pc, either direction. Yet both pcs can
connect to every other pc on network, access the shares etc.!!

As a workaround I'm copying files thru a third pc or if files big enough to
justify I instead just use portable hard disk.

I've been at this long enough (since 84) to notice anything unusual but I
can't see anything suspect. Protocols and settings all appear as expected
from all the pcs. etc. etc.

Any ideas on where effort should be spent?