Why use a PC at all?

If you're going to hook up a PC to a display, at least make the display alphanumeric or graphic.

If you want to experiment with basic circuitry and use your PC as the flexible control, then the best approach is to omit a microprocessor and serial port and merely use the parallel printer port to latch some characters into your Numeric 7 segment displays.

Give us more direction about what you are trying to do and maybe when can be more helpful.

aka j