sixca has compiled nice example on how to control devices via USB interface. Simple solution is to take microcontroller with USB interface built in. PIC15F4550 has full speed USB interface with full speed capability. Microcontroller uses on chip USB driver which is pretty easy to program.
Firmware is programmed compiled with MPLAB C18 compiler tools. You may want to edit source in order to interface different devices like LCD, relays etc. PC software part is written with Delphi 6. It has three commands implemented: “Turn Left”, “Turn Right” and “Stop Motor”. This software can also be modified for your needs and recompiled with Delphi6 compiler.