I would reccommend NOT using regular computer speakers in your car. There are many threads about that and pointing out why NOT to use them. Basically they will sound great in your house, and they will sound tinny and drowned out in your car. Car speakers are menat to work in the car overcoming road noise and the ackward angles they are placed in (at your feet in the door).
If you don't want to buy an amp, then the cassette adaptor is probably your best bet. Otherwise just get a cheap 4channel amp and send the signal straight to your speakers. Then a mono for your subwoofer if you have/want one. The center speaker is almost useless, and you will not be able to tell the difference. If you really need it, get another speaker for like $30 to $40, and mold it in your back seat or something.
So in short, do not use the PC speakers... Do a search for it, and you will know why.