For a regular (amplified) subwoofer, you run rca cables from your receiver to the LFE inputs on the subwoofer.
In you situationr, provided the card in your computer has LFE (or subwoofer outputs), you run the rca cables from your PC to an amplifier (which you must purchase) and then run from the amp to the passive subwoofer. If you have an old receiver sitting around, you might be able to use it. I have used an old receiver after the stand-alone amplifier for my passive subwoofer died the second time. You could also look at the small amplifiers at Parts Express, many of which should be adequate for your purpose.
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