That was my goal... 
I suppose it depends on your processor and if you have small amount of RAM available - but I've found that its active processes especially drawing, animation slideshows, encoding/decoding videos etc are top of the list. But if they are not actually running disabling them won't matter, I wouldn't think. What seems to matter is if you configured a lot of automatic processes like I said : things that come to mind are autoimport running in the background (add importing tags and artwork and things like that can get pretty intensive (comparatively speaking), syncing over the network, amongst others -- even if they are low priority processes.
Doesn't seem to matter for me though with an i5+8go Ram +Win 7 if I use 11% of the CPU its pretty much the max.
I'm interested in this more because I'm tinkering with building a low power (PSU 60 to 120 watts) for a silent PC with a tiny motherboard using my TV as a monitor.
