Windows Vista throttles your network bandwidth when you are playing any multimedia file such as an MP3 or a video. This was designed to ensure the CPU has enough free cycles available to play your media without skipping. The down side is that when you are playing a multimedia file you will notice that your network speed of file transfers will decrease on high speed network connections. By default when you are playing a multimedia file your network transfers are limited to 10 packets per millisecond.
In Windows Vista SP1 Microsoft introduces a registry key that allows you to customize this setting.
You can set NetworkThrottlingIndexValue between 1 and 70. If you want to disable set the hexadecimal value to FFFFFFFF.
Microsoft warns that if you increase the value above 10 you may experience playback quality issues. Depending on your network setup, it is worth experimenting.
After making any changes a restart is needed.
Read more about this tweak on KB948066