DavidR
3
There has been something in the topics relating to a bad jollyroger.vpu which contains the banned registration keys, whilst this is related to updates failing because of an invalid key, it may possibly be involved in the validation.
However, I would have thought it would have reported the key as invalid on input if this were correct, check out this topic and try what is suggested about downloading a new jollyroger.vpu, http://forum.avast.com/index.php?topic=27185.msg222148#msg222148.
@ mrcoffee
Have you checked your system date particularly month and year ?