spdif output from a laptop?

I need to get my laptop to connect to my home stereo DAC via spdif but it doesn't have the appropriate output.

Being digital, is there any software out there that will allow me to output via firewire or USB ports with a correctly wired cable? Or, and no doubt more expensive, is there a pcmcia card that will do the same thing. I only need the output for mp3 as the laptop will become the centre of my stereo (it's so much quicker than finding the damn CD!)

Andy