Fan Aircon wont turn off

Hi all

I have a 2002 320d the aircon/fan wont turn of once you turn it on the only way to kill it is to disconnect the battery.It also turns itself on everytime you start the car,if you turn it off immediately it will go off but leave it on for more than 5 secs and you're back to the battery solution.Also the fan will not run at a constant speed if you decide to leave it running.

The agents suggest a new computer box.any cheaper alternatives.

Thanks Fanie