Playing your radio speaker aloud and letting the computer microphone pick it up can work surprisingly well, especially in modes that are already hardened someone against noise and interference like Olivia. That will get you started receiving, from there you can decide if you want to build an interface.
On 2 meter FM, we've successfully exchanged slow scan pics using MMSSTV and just hand mics and speakers with a nearby laptop. On HF SSTV, PSK31 and other modes, I used to let the laptop in my van copy them while I was driving just by reading them in with the laptop mic off the regular radio speaker - if it works in that noisy environment (and SSTV did show the noise, but came through surprisingly well) it will work around a quiet desktop and I've had several friends get started that way.
Check out
http://www.qsl.net/wm2u/interface.html. He's got some simple schematics of how to make basic interfaces with just a resistive divider. With an isolated laptop, that may be all you need.
For PTT, they often show an opto isolator or a transistor switch. For myself, what I often do for simple interfaces is get a cheap reed relay from the junk box or off the wall at radio shack. Put it in series with a simple diode, 1n4148 is adequate, on the CTS or RTS line to ground so that when the line is asserted (flips polarity) it will close the relay contacts and there you have a nice isolated PTT connection. (when the line is not asserted, it will be the other polarity somewhere between 5 and 15 volts and the diode will block the flow and the relay opens). It might not be fast enough for some really critical modes like AMTOR or Pactor, but it's fine for RTTY, PSK, SSTV, Olivia, etc.
My favorite program for PSK31 is digipan 2.0. I also enjoy listening and copying MFSK, it has a nice rolling sound to it, sort of reminiscent of RYRYRYR on rtty.
Enjoy and let us know how you make out.
