PC -> stereo:
Figure out if your stereo has some sort of aux input. Most do. If you want to connect via analog, you get a stereo minijack to <insert stereo connector here> cable and plug the stereo minijack end into your PC speaker output, and the other end into the stereo. A common analog connector on a stereo is RCA, which uses two connectors. The cable should be about $5.
If you want to connect via digital, which will probably give the best quality, you have to figure out if your sound card can output digitally. SBLives can. I don't know what kind of connection it uses though - the manual should have something about it. Once you get the cable and have it hooked up thru the digital output, you might have to change a setting to tell it to output digital, but probably not. Then on your stereo you select the input to play with your remote, and viola! instant home theater.