The typical line input on a PC is a 1/8" (3.5mm) stereo mini-plug, so you need a cable and any necessary adapters to connect the output of the stereo to the line in of the PC. The most common cable for this is a 1/8" to 1/8" connector. Then, if the headphone jack is 1/4", you would need an adapter. Many headphones come standard with a 1/8" plug and a 1/4" adapter provided. This adapter would work just as well for connecting the headphone jack to a PC input.
A headphone jack is not as ideal as an AUX jack because a headphone jack has a variable level controlled by the volume. When using a headphone jack instead, you will need to adjust the volume level so that the output of the player does not overdrive the inputs nor should the signal be low because that will increase the noise.