Here is a great technical article why and when this can happen. In short ,when you play multimedia like sound or video Multimedia Class Scheduler Service (MMCSS) makes sure that multimedia get priority.
"if your system is on a 100Mb network, you typically won’t see any slowdown. ... If you have a 1Gb network infrastructure and both the sending system and your Vista receiving system have 1Gb network adapters, you’ll see throughput drop to roughly 15%. ... Internet traffic, even on the best broadband connection, won’t be affected" Mark says.
PS. Process Explorer is now v11.02 and works finally great with Windows Vista.