What I got from your post is that:
1) You have stepper motor board which is designed to be interfaced to a PC parallel port.
2) Due to MS Windows timing issues, you don't want to drive it from your PC parallel port.
3) You want to interface an 8051 between your PC and the stepper motor card.
4) You will send commands to the 8051 (Over COM port)and this 8051 will emulate a PC parallel port.

Am I right?

An 8051 can emulate a PC parallel port (speed may be an issue if you need high speed emulation). Even if you want to emulate the parallel port on 8051, you must buffer it before you connect it to your stepper motor controller card.