you probably need a stereo component in the middle. if the speakers you have are from a stereo, hook them up there, and then use a cable to connect from the pc to the stereo. if your stereo accepts rca cables for aux-in, you can find a transformer from 1/4" to RCA at any electronics store for 5 bucks.
going directly might not work because of power differences(music inspeakers would be too low) and the fact that it's a pain in the butt to add a 1/4" connector to thespeaker cables.
more experienced users might have other solutions.