That is a pretty advanced topic. Interfacing with the ECM is a pretty car-specific project.
Keep in mind that if it is too early a model, you will be working with OBDI rather than OBDII.
Just a thought, if you can figure out the inputs to the odometer LCD itself, you could 'read' the figures on it and relay those to your PC. It's beyond me how to do this, but I'm sure it can be done.