Well, you first need to see what voltage the speakers use. If the speakers use 12 volts, then the car battery puts out 12 volts. Otherwise, I'd use an AC power inverter, a device that allows you to convert a car's 12v power into standard 120V AC wall power. You'll want an inverter capable of handling the wattage of your speakers though.
I'm not sure why you'd want to use computer speakers in a car, but that's how I would do it if I had to.