i just used a home reciever and plugged RCA's into my comps speaker input and then i just hooked the sub up into one of the channels.. good ole 10" Pyramid Pro Plus in a bandpass box first sub i ever had, and it was ok for computer usage
You can buy 4 or 5 speaker setups that have subwoofers with them. You can get them pretty cheap at Best Buy or online someplace. One of my friends has a 5 speaker system with an 8" subwoofer under his desk. That thing screams. I think its a lot easier to do that then to mess with hooking up a car sub to a computer...