Sending packets but not receiving

I have several pcs on a static IP addressed network.
Serveral of these machines have Windows XP Pro and will do
the same thing. These machines have been up and running
for over a year on the network and accessing the internet.
For some reason some of them will not get on the Network
because they can send packets, but not receive. Most of
the time I can connect a dumb hub between the switch and
the computor in question and it will fix it. And serval
days later I can take out the dumb hub and it will still
work. I have tried new nic cards, reformatting the
computer, checking all the settings I origanlly used the
get them on the net, and yes the XP firewall is shut off
even though other same pcs will work on the network with
the firewall activated. Anybody got a clue ????