Between my Gigabit LAN connections, I see 250-400 Mbps (31 MB- 50 MB) transfer speed.
Antivirus, especially McAffee, gets in the way of that transfer with on access scans.
Defraggers, get in the way.
Open up task manager and watch what goes highest CPU, it should be your transferring program.
The only way to get the max out is to have one read disk access transferring to one write disk access.
I found using FTP, instead of Windows network, will transfer faster. Make sure allow only one connection at a time.
Since it is Windows 7 (assumed all machines), it should be optimized for broadband, so that shouldn't be a problem.
If not all W7, use TCPOptimizer on the non-W7 machines (www.speedguide.net)
Update all NIC drivers.
I hope this helps