This robot has no "brain" even though it has a Arduino processor.
The brain for the bot is located in the PC and the program is S4A (Scratch for Arduino, version 1.5) based on Scratch, a MIT developed block programming language easy for early programmers (and even children) to learn.
The "tail" of the bot is a permanent USB umbilical providing commands to the Arduino on the bot and all power to the bot, including all motor power. With a "light weight" USB cable it works out fairly well.
Right now, motor control is speed and direction only provided by dual 2N2222 transistors for each motor. (Reverse could be added using any common H-Bridge chip, but a low voltage, low current consumption chip would work out best.)
This was a fun robot to build and would (combined with the S4A software) make a great learning environment for both robotics and programming.