As a suggestion, research what application software you are planning to use or create, what interfaces you're comfortable with, and then ask a specific question.
How do we know the answers to these questions, and indeed even what interfaces your PC has, and what OS it is running, and what skills you have?
All we currently know is that you don't know about microcontrollers.
Sorry, I don't have source code and a circuit diagram.
As a suggestion, if you don't want to use a microcontroller, you'll find life hard with a serial port.
If you use a parallel port, life will be difficult too - you'll need to find a compatible library for XP
and you may need to make some BIOS settings adjustments to get the parallel port working, and
it may not be forward-compatible with more recent OS's like Windows 7 and 8.