Here's the easiest way (I think):
Make a matrix (24 by 24 = 576 LEDs). Then, say LED #10 should be turned ON. If it's located on "input" 2 and "output" 5, then make "input" 2 High and "output" 5 Low. Keep other "outputs" high and other inputs "low".
You'll need to use 6 8-bit multiplexers (3 for "input" and 3 for "output"). Their addresses will be controlled to the PC parallel Port, so you'll need:
3 outputs for address selection on "inputs"
3 outputs for address selection on "outputs"
2 outputs for chip selection on "input" (1 out of 3 chips should be selected)
2 outputs for chip selection on "output"
After that, you can connect (in parallel) the corresponding address selection pins on 3 "input" and "output" multiplexers. The chip selection will tell to multiplexers which one the PC is controlling.
The LEDs should have their + connected to "input" multiplexers and - to "output" multiplexers. The Input of each multiplexer should be tied to Vcc.
Good Luck,
TI|CP