Actually its not hard at all. This summer I've been retro fitting a small gas powered vehical to be controlled by an x86 PC. The PC is controlled by another PC over a wifi card. The networking stuff is kinda complex, however the parallel port is great for controlling realys and reading data from sensors like limit switches. The code for that is very simple and not hard to learn. If you are adventurous then there are pretty cool microcontrollers that can be connected to usb. Also, there are many serial options like the basic stamps. For my robot I have
designed my own realys because I did'nt have the cash for Spikes. Also I am using a microcontroller that connects to the usb port that way I can connect many microcontrollers and therefore have countless sensors and motors. If you want to see code I would suggest reading the following, all of my code is in C and C++, also I am using Linux but Window$ could probably allow you to do much of the same things. All bets are off when it comes to Macs, not really but options become limited by the ports on them.
http://www.phrack.org/phrack/50/P50-11
http://www.epanorama.net/circuits/parallel_output.html
My parallel port test program(Very messy)