Weird update behaviour

What’s wrong with that stupid Anti Virus program?
Updating takes ages so I’ve investigated what’s going on.
It appears that the file bases.cav shows very strange behaviour.
While updating the filesize constantly fluctuates between 0 and 96 MB.
When it has reached its maximum size it suddenly drops to 0 and then it grows again.
This goes on for about 20 minutes or so and then CIS has apparently decided that the file has been updated.
On the next update cycle the process repeats itself again and it takes again about 20 minutes before updating is completed.

B.T.W.: I did a clean install (6 times to be precisely) and that weird updating behaviour appears to be reproducible.