There are two ways that you can do this, actually.
1) If you have a 2.1 speaker set, hook it up to the front speaker jack of the SB Live. Get an 1/8"-stereo-minijack-to-RCA cable and hook up your stereo to the rear speaker jack of the SB Live; or
2) If you have a 4.1 speaker set, get a 1/8" stereo minijack splitter cable and an 1/8"-stereo-minijack-to-RCA cable. Hook up your stereo to the front speaker jack of the SB Live via the splitter, then hook up your computer speakers normally.
Hope that helps,

atwl