You can buy an inexpensive OBD-II / RS-232 adapter from Multiplex Engineering:

http://www.multiplex-engineering.com/

This device enables your PC to communicate directly to your car's PCM through the PC's RS-232 port. I own one of these adapters. It has saved me hundreds of dollars by enabling me to do diagnostics on my own car. However I did not buy my adapter directly from Multiplex but from a guy who was selling software that drove the Multiplex adapter. Unfortunately that person is no longer in business (past away?). After buying the adapter, I was intrigued by the whole process and began writing my own software (the software I originally bought was only an executable). I now have something that will communicate with most of the SAE documented OBD-II protocols.

Getting my software to work was nontrivial, e.g. I had to do all the RS-232 calls at the BIOS level while running under DOS. I've tried to get my code to function under Linux but unfortunately it runs glacially slow (haven't worked out the time slicing right). My biggest problem with this adapter has been reverse engineering the proprietary Saturn PIDs (Saturn PCM command codes). I have a disembodied PCM that I pulled from a wrecked 1999 Saturn that I use for code development. Some of the PIDs will actually cause the PCM to shut down (booby trap codes?). After doing that, the only way to get the PCM to wake up is to cycle the power. I would not dare do this with the live PCM attached to my car.

It's a fun hobby thing to do (a bit like crossword puzzles).

Gary