Windows Vista throttles the network interfaces while audio is playing in order to maintain audio quality and prevent network interrupts interrupting media playback. By default, this limits the network receive speed to 10% of its maximum - 100Mbps on gigabit ethernet, and 10Mbps on fast ethernet - but since Vista SP1 you can alter that percentage by changing a registry value: HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Multimedia\SystemProfile\NetworkThrottlingIndex Change it to a value between 1 and 100 (and then reboot) to set the maximum downstream network bandwidth that can be used while playing audio. Not an ideal solution, but better than having none at all…

http://www.anandtech.com/systems/showdoc.aspx?i=3233&p=2