Sounds like MTU.
Try changing to 1400 on one of your computers, or if you have the option, the best place on on your router.
In windows 2000 and XP:
Open Regedit and find this key:
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
You will need to create a new DWORD entry:
MTU="1400"
If this works, you can tweak the number to try and get more speed.
