Arduino motor shield lacking power
There is definitely something wrong here. Look at this page: http://www.maplin.co.uk/small-motor-3738#specification


  Operating voltage:    1.5 to 3V
  
  No load current:  200mA
  
  Current at max efficiency:    660mA


I bet the motor shield outputs a higher voltage than 3V and given these specifications the current draw from the USB port was probably exceeded.

Check your Arduino and motor shield on another USB port without the motor connected. (BTW: I sometimes have to reboot my PC when my USB port went into overcurrent protection).

In future use an external power supply when powering motors as the current they draw it too high for the average PC.

Also connect a series resistor to reduce the motor voltage to 3V or even better: use a 3V power supply or a 5V motor.