It looks like Timmy may have gotten a solution already.
I suggest getting the Asus Xonar 5.1 channel PCI card for $30.
It will boost your quality. crisper sounds, deeper and more present bass, better contrast and less blending of the different frequencies.
The cable you will need, to my understanding is $3 on amazon ( eForCity 3.5mm to RCA Auxiliary Cable Cord for iPod/MP3 ).
It will have a 3.5 mm male end to plug into your computer and 2 av male ends to plug into your speaker system.
The sound card will connect to the headphone jack on the front of your pc (if you find the wire and plug it in). Then you can plug your speakers into that jack.
OR you can plug them directly into the card on the back of the PC, I believe you would use the "Front" labeled jack.