The same company that sells the item mneary linked to above also has a how-to guide to buidling their 12 foot digital wall clock:
SparkFun Electronics
That'll get you 75% of what you're trying to do. Instead of the GPS signal, you'd probably just use binary counter ICs. As for the computer control side of it, the "outb" and "inb" functions in c++ will give you control of the pins on your PC's parallel port. Google "parallel port programming".
I recently built a large digital clock similar to the SparkFun design to use as a race timer. I can help with some of the specifics if you run into any roadblocks.
-Dan