It's been a while since a bug like this has been found in Windows. Usually they are security flaws and virii, but this is a bona-fide bug. This applies only to Windows Vista.
It appears that listening to music or watching videos in Vista results in an almost immediate drop in transfer speeds with some users seeing their network cards utilize only 5% of a gigabit network device. The same users reported 50% usage under the same circumstances in Windows XP.
Microsoft is working on a fix, but for now, if you're downloading
bootleg movies legal Linux ISO's, turn your music off.
StreamingMedia.com: Vista Network Degradation Bug Linked to Streaming