I don't know where the bottleneck is. The slow down might not be on the network side. IF this is a network problem, then there are a couple of things you could experiment with.
First, I would make sure the switch is using the latest firmware. I have not looked through the notes for the firmware upgrades, but if you have version 1 of the GS108T switch, I see many firmware versions that have been available for download.
Second, I should make sure all link partners (switch ports, server, and desktop) are configured the same for speed & duplex. Usually automatic negotiation works, but you could always try forcing speed and duplex to see if that makes a difference.
Transferring large file sizes might benefit from configuring the network for jumbo frames. Your switch supports jumbo frames. Unfortunately, the Intel(R) 82578DC Gigabit Network Connection does not support jumbo frames, so you would have to buy a plug-in adapter to get that capability. The Intel(R) Gigabit CT Desktop Adapter supports jumbo frames.
You might want to try tweaking some of the advanced network settings in the Windows device manager properties. If you installed Intel(R) PROSet, then you will see help for each of the settings. If you have lots of system RAM, you could try increasing receive or transmit buffers or tweaking some of the other performance options. You could try enabling Large Send Offload. Changing advanced settings could help or make things worse depending on what is happening.
Of course, none of the above will matter if the slow down is not on the network. Maybe someone else might have other ideas.
To reduce network latency, disabling Nagles Algorithm will often make a difference. It is common practise amongst gamers, and where the primary purpose of a machine is copying files. I've done this myself between 2003 servers and in XP, but not so far in Windows 7. A quick Google for help though seems to indicate it is possible in Win7. As always though, research carefully first as it does involve editing the registry and find an instruction set you are comfortable following.
I've noticed the same problem, when my RAID5 is rebuilding or writing at full speed, my 82578DC NIC drops to 20~30MB/s maximum speed
Fedora13 with kernel 188.8.131.52-56.fc13.x86_64
CPU: i7 firstname.lastname@example.orgGHz (stock)
2x4GB DDR3-1333 (stock)
P55 on Biostar TPower i55
5x1TB Western Digital Green (5400rpm)
1x1TB Hitachi (7200rpm) - system drive
Right now, the system is experiencing this problem, reading 320MB/s from discs, Rebuilding at 60MB/s write on the parity disk:
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdc1 sdd1 sde1 sdf1 sdb1
3907035136 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/5] [UUUUU]
[=======>.............] resync = 39.0% (381681536/976758784) finish=168.9min speed=59710K/sec
latest drivers and kernel, the system is responsive but the network is 0.400ms delayed with no traffic.
It seems that it is a bottleneck on the DMI itself or the ICH10 controller is reaching its peak.
I'll try to route my Gib storage net to the Realtek on board to see if things behave better.