A laptop is simple. I have used mine to play MP3's for the last 3 years. I went to radio shack (yes radio shack) and picked up a 1/8th inch male stereo jack to left and right male RCA jacks. My laptop (IBM thinkpad) has a stereo out female jack. Then you plug the 2 RCA's to the Line in RCA female on your headunit.
Dell laptops have RCA outs that can be plugged via a set of RCA cables. Many new headunits also have an auxiliary out in the front or back of them. Depending on the equipment yes this is easliy done for $3....