Now,my eml has come on with de pollution fault(same as anti pollution on the 406),but the car is driving better than it has done,ie much better on aceleration

So i fired up the pc tonight to check the fault,but the lexia box wont stay lit up.When i plug it into the usb port,the green light comes on for 3 seconds,then goes out.I connected it to the 2nd usb slot,same thing,even after "found new hardware"and inserting my disc etc.
Anyone have any ideas please??