I had this problem on my PC
with a SoundBlaster Live! card under XP
. Tried updating all the drivers etc, but still no luck until I bought an Audigy 2
I know Creative and some other companies do USB soundcards, often with more ports too if you're into that kind of thing - I guess you could pick one up for around £40 (off the top of my head.)
Another possibility is the MP3's are poorly encoded because I know I've had some files that sound fine and then suddenly scratch, hiss and pop all at once, scaring the shit out of me! Guess I'm asking does it occur with the same songs at the same point?