A question that frequently comes up on some of the various radio mailing lists I'm on is how to interface the radio to a computer for doing digital modes via the sound card. The sound card provides the equivalent of a modem, doing digital to analog and analog to digital conversion.
Skip Tellar, KH6TY, has a design called DigiVOX that was in the March 2011 QST for sound card interfacing. It will do a VOX function for activating the Push-To-Talk for doing digital modes without using any additional signaling lines.
Mark Phillips, NI2O, sells a kit called USB Digivox that combines a USB sound
card and KH6TY's digiVOX interface at his Go Box Gizmos site. This simplifies the setup and gives a dedicated sound card for the radio analog to digital and vice-versa.
Argent Data systems is a good source for a microphone/speaker cable for connecting to Kenwood, Wouxun, Puxing, and other chinese radios that emulate the Kenwood.
The software that is the most popular for doing sound card digital modes is FLdigi by W1HKJ. It can do quite a few different modes and is cross platform. It's got a lot of functionality and though has a bit of a learning curve. Fortunately there are a lot of tutorials out there on how to use FLdigi.
Note: Laptops often have a MIC input connector and not a line level input. On the ring of the MIC connector is 2-5V for biasing the microphone element. You want to take care with this, I'd at least make sure to put a capacitor to block the DC or make no connection to that side. If I remember correctly it corresponds to the Right (Red) channel.