Your problem could be one or a combination of things.
First, if you have onboard audio such as AC97, playing an mp3 or other compressed format takes cpu time away from other processes. If the mp3 is not processed promptly it's going to be poor quality. I just installed a sound card for this reason (not for any audio benefit).
Many online progs such as filesharing and leeching seem to hog the pc. Even if you set the process priority to low you can still feel the pc is sluggish. The amount of processor power used is minimal but somehow the progs are hogging the interrupts (or something similar). You may have a conflict between Firefox and Limewire trying to be top hog and the music is the loser. Firefox is normally very low profile. Per fink's suggestion, try lowering Limewire's process priority.
Starman*
~
~ if all else fails, read the instructions