Well, I tend to use my standard speakers as surround speakers and my stereo is hooked up to the main output. I use a 1.3mm Jack to Audio phono converter leads, which are pretty easy to make, and pretty cheap to buy, but since I use two computers for my audio work, I have to put it all through a multi-patch box.
Basically, the audio out from your computer goes into the switchbox, and the first audio out from the box would go to the Hifi, the other goes to the computer speakers. Word of warning though, you will lose volume if you divide up the signal like that, a pre-amp is advised, but is not vital.