Windows Vista uses a new service called  Multimedia Class Scheduler Service or MMCSS which runs under svchost.

This new service is used to boost performance when a multimedia application begins playback, the multimedia APIs it uses call the MMCSS service to boost the priority of the playback thread into the realtime range. This creates for a seamless playback without those annoying glitches in music when you are doing heavy duty computing at the same time.

Tests of MMCSS during Vista development showed that, even with thread-priority boosting, heavy network traffic can cause enough long-running cpu interrupts to prevent playback threads from keeping up with their media streaming requirements, resulting in glitching. MMCSS’ glitch-resistant mechanisms were therefore extended to include throttling of network activity. It does so by issuing a command to the NDIS device driver, which is the driver that gives packets received by network adapter drivers to the TCP/IP driver, that causes NDIS to “indicate”, or pass along, at most 10 packets per millisecond (10,000 packets per second).

This loss can be up to 15% on a gigabit network.

The Tweak:

Now you can't just disable the MMCSS service since windows audio is dependent on it. In other words disabling it means no sound for your computer. 

So I ran regedit (type regedit in the start menu search box), and navigated and changed the key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Audiosrv\DependOnService

Now remove MMCS from that key in the list, and set MMCS to disabled in services (type services in the start menu search) Now reboot!

As soon as I rebooted I was able to copy files at 40mb/s+ while listening to audio

Example of priority boosting by Vista:

Example of network throttling: