Picked this up in another tread:
For those who would still like to get rid of the annoying "NAG" window for version 5 update, do the following:
Disable the self-defense module.
Locate and rename the file "asw5Not2.exe". It should be located in the Avast4 folder.
Re-enable the self-defense module.
You'll now have the monumental task of remembering, yourself, to update to version 5 before version 4 expires.
This was apparently part of a virus definition update, even though it has nothing to do with virus definitions.
Also you could just block the "service" (an annoying one) in your firewall.