People over at the 2CPU Forum have found a bug in Windows Vista which causes a big network performance drop when you're playing music on your system:
Over the months since Vista's release, there has been no doubt about the reduced level of network performance experienced compared to Windows XP. However, some users over at the 2CPU forums have discovered an unexplained connection with audio playback resulting in a cap at approximately 5%-10% of total network throughput. Whenever any audio is being sent to a sound card (even, several users report, while paused), network performance is instantly reduced. As soon as the audio is stopped, the throughput begins to climb to its expected speed.