If you are getting that 1 hour thing from the windows explorer dialog when you start copying - it is horribly unreliable. doing pings between the machines should help.
Is it a hub or a switch? a hub is really quite inefficient in how it networks, so it may be worth replacing with a switch...
Explanation of difference between switch and hub
edit - doh! what bendsley said
Also, if you have firewall software it may be possible to see which programs are accessing the network - this can help tell if you are being flooded by something...