Hi,
> I believe you're thinking of the episode
"Face of Evil",
>which introduced Leela as the Doctor's companion.
No it was the one with the giant flies and the 3rd Dr.
Evil computer, giant flies, 3rd Doctor....I've seen just about every episode
of Dr.Who but that doesn't ring any bells. Doctor #4 encountered the
wasp-like "Wirrn" in "The Ark in Space", but evil computer...?
Have a look here:
http://www.bbc.co.uk/doctorwho/classic/episodeguide/. I'm
sure you'll find your story.
TTFN - Pete.