Whenever I see something like this, I think "TCP Offloading again?!". Might not be it, but it's easy enough to try. Receive Side Scaling is another possibility. You can do this right on the NIC in Device Manager or via the Registry. The articles below show how to do it both ways (note that the symptoms the second describes are a little different, but really, anything with the words "slow" and "network" are enough to think TCP Offloading and/or its cousins).
Thanks for your suggestion!
I have tried disabling RSS and TCP offloading using the information in the links you provided, but to no avail. I have also tried setting the speed down to 100 mbps full duplex - again no difference.
I'm thinking I may try to track down a wireless USB adapter, disable the on-board NICs and try using the wireless adapter.
Thanks anyway, I'll keep plugging away.