I am an avid user of Avast but for the last 3 days I can,t update the virus definitions.
Firstly, the update activity is excruciatingly slow. A 5 or 7 Mb download takes nearly one hour against the 10 minutes required with the internet speed that I have.
Secondly, at the end of nearly one hour, the update terminates with the error of invalid signature without completing the update.
Wll downloading the update file and running it bring the system up-to-date ??
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair).
If this does not help, can you uninstall / boot / install / boot again?