So much depends on the player and codecs. Some codecs require tons more horsepower to decode than others, for instance. Some players take advantage of hardware overlay while others default to software emulation. Also, if your media is recorded at (say) 640x480 and you're stretching it to 1280x1024 that requires additional juice.
To complicate matters further, Vista introduced a new model for the desktop window, so if you're using an older player it's being forced through additional hoops to get anything on the screen.
So what kind of media are you playing back, at what resolution, and with what player? I personally can't hep much since I'm an idiot about video nuts and bolts, but anyone who knows their stuff will require at least this information to start with.