The format of the headphone jack and MIDI is totally different and are incompatible.
But does your laptop already have MIDI Out in the first place? Note that some audio chipsets use the joystick port for MIDI In/Out, so you just need to get one of those MIDI cables that have the 15-pin joystick port on one end and MIDI In and MIDI Out at the other end. Check your laptop to see if it indeed already has this functionality.
If not, then you will need additional hardware (either the Creative Extigy or some PCMCIA solution) for your MIDI needs.
Hope that helps,
🙂atwl