What avast version are you using, avast 8.0 or 9.0 free/pro/ais/premier ?

Avast 9.0.2006 is meant to be win8 compatible, avast 8.0 has been reported to be removed when updating from win 8.0 to win 8.1, so I don’t know if this might have something to do with the message you are getting.