Playing mp3s encoded at 192kpbs, I'm showing between 0 - 2 & 4% CPU use, same with mpg video/audio as well as standard youtube video/audio. Playing a commercial DVD shows approx 2%. Don't know about running a torrent prg, don't use those.
I use this machine for multitrack audio recording so I've taken some extreme measures to optimize realtime use, much of it not advised for general purpose use. Basically, I try to keep OS interference to a minimum, this includes background tasks and services. (I've gone back to running Areo without any penalty) 10 Windows speed tips that DON'T work.