Have you checked Windows firewall on all of these machines? I would also check the LAN connectivity by pinging the server (in this case Vista Business) via the command console to check for packet loss. I.E.:
Run -> cmd
ping -n 10000 IP address of Vista.business share
You could also try this from the server to the IP of each machine that is having difficulties, thus eliminating the possibility of any problems with the physical connections to these PC's.
If you do receive packet loss you may have faulty cabling or a bad router.