Windows 7 is network friendly, what this means is that if you're using a media player or voip type program it will throttle other applications running on your computer. I've seen network transfers on my gigabit lan throttled to 30 KB/s when using Winamp at the same time.
There are some references you can read for more information to see if this is whats causing your problem
http://www.speedguide.net/read_articles.php?id=2574
http://technet.microsoft.com/en-us/library/bb878127.aspx
This may or may not be a factor but its a very good starting point.
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