connecting hi-fi system/speakers with computer

my hi-fi system has these great speakers, my computer's are crap. These speakers are connected via 4 wires each (maybe they include subwoofers ?). My hi-fi system has no AUX connector, only FM and AM antenna. And a headphone out, which could easily be connected to the line in of the computer...

It seams a shame to buy new speakers when I have these brilliant ones right here. Any ideas on connecting the speakers/whole system to the computer ? Are there (cheap) sound cards that I could connect such speakers to ? FM streaming could be difficult since the system is connected to the roof antenna, and I still want to use that. And I don't know about the legal situation when it comes to strong signals (especially when playing GTA...)

Can you take a picture of the back of the HiFi system? I'm not understanding what connections you have. Usually they have a red/black pair of wires or a new optical audio jack.

detailed description: (no camera handy)

per speaker:
gray/blue: +/- high 6Ω
red/black: +/- low 6Ω

it looks like the high and low sounds are handled by two different integrated speakers.

otherwise: AM antenna (two wires), FM antenna (standard radio/TV antenna connector)