MP3 vs CDs on Laptop

[Funambulistic]

Perhaps someone can explain this to me (I am in no way a “technical” person and rely on my rather subjective ears). I have been a subscriber to Emusic.com for a while – not for “audiophile” music, but to broaden my musical horizons. Of course, everything they have to offer is MP3 (not sure of the bit rate, but it is rather low – 128?). Everything sounds pretty good via my iPhone (or iTunes on my laptop) through my Nuforce NE-700M ‘phones. I downloaded Ani DiFrancos’ “Up Up Up Up Up Up” and it sounded so wonderful that I went out and bought the disc. On my two channel system (Arcam DV137, T+A Powerplant, Sierra-1 [with the last NrT upgrade before the next shipment – hehe]) it sounds fantastic (speaker review to follow). I have since had to put my system in storage and have been relying on said IE headphones. Now to my quandary: On my Asus laptop (out of the box – no soundcard upgrade or anything like that) MP3s sound better than CDs via the built-in player (case-in-point – I would rather listen to Ani on iTunes MP3…). Is this a result of the built in DAC? Would I be better served to purchase an inexpensive USB DAC/Headphone Amplifier to bypass the internals (Nuforce has one – I am sure there are more out there) for my cans listening (I do plan on getting a dedicated one for a future computer based system so I can revel in the wonders of 96 or 192 rates in glorious Sierra-1 NrT sound) or is there a magical setting I am not aware of? BTW, I am running Windows 7 (would rather be a Mac user, but $$$...). Thank you in advance for any response/input/advice.