Not sure if Parallax has them (12F or 10F) on their boards.
So yeah, MR_E went right over your head.
Does your PC not have fan speed control? You can get PC fans now that do this and play a little sound when they are too hot.
What you need for the project.
Input and Output.. I/O.
You need to know the temp (input). Google for thermistor.. That part will with a circuit tell the board the temp. Also see Nigel's joystick tutorial if you are running on battery.
Now that the board knows the temp. You need an output circuit to make the fan go faster or slower..
Let me know if this helps. But they make smart fans now. With a pot you can set and everything..