Since conforming to iPodism I've been using iTunes more, however, before when I managed my music folders myself and most of it was .mp3, I had a simple little free program called Destiny Media Player that I picked up years ago.
Small, compact and the functions you need and little else, and uses about as much resources as notepad! So good that I've kept it all these years. I used to be listening to music constatly in the background with it, had no effect on the PC at all.