I do recommend a new full download of avast! setup.
Desinstall / Boot / Download and run avast 4.0 Uninstall / Boot / Install again
Try to update and see what happens. This is the text of the update of today (November 5th):
New iAVS update for avast!4 and avast32 has been released recently.
Related files could be also found on our Internet sites.
Note: This is a standard update