If it is necessary to use serial communication you may need a microcontroller based system. Else if parallel port can be used, the whole thing becomes very simple. You can directly control the stepping motor from printer parallel port.
I don't work with 8051 but you can use the PIC 18F4550 mcu for interfacing with a computer via USB. The code is well documented in the datasheet..you probably have to implement a serial to USB converter cdc...
I use CCS compiler, and the code for implementing the USB driver is provided, similar driver codes are there for Microchip's C compiler, you just need to study it..
A good book on USB interfacing would also be helpful..
simply interface the dc motor to the controller using the H-bridge..
communicate with the controller using any of the terminal communication software..
now when you recieve any thing from the serial port of the pc to the rx of the controller, just check if it is 'A' or 'B', compare them and do whatever you want i.e. rotate the motor in clockwise or anticlockwise, stop it or actuate it...