your sustaining 40Mb/s, roughly half of the network peak.
with Windows machines, I find they rarely sustain over 50% of the throughput, whereas Mac and *Nix usually sustain about 75% or so of peak throughput.
there are some reg tweaks to increase the throughput of the TCP stack in Windows, but I dont have any links on hand
fyi, no matter what system you use, you will never sustain peak throughput on a network(unless your throwing thousands of $$$ into high-end interconnects on a gigabit LAN)