Seeing Vista PC from networked laptops

In Vista on PC with ethernet link to radio router, in "Network" I can see the
router, and all computers on the network (the PC and two laptops). In
"Sharing and Discovery" - "Show me all the shared network folders on this
computer" shows C: drive, Gexternal) drive, printer, etc. The PC can get to
the shared folders on the laptops running on XP.
However the laptops cannot access the PC and get at the shared folders.
The Vista PC can ping both laptops but the laptops cannot ping the PC or
each other.
Any ideas gratefully received - I'm worn out!!