I've had this same error, quite a while ago. My (somewhat vaguely) remembered experience, plus a Google search lead me to think you should try a reinstall.
Firstly I should point out I am not a guru, and am posting this because a guru has not replied here yet.
If a guru should happen to see this and disagree with my suggestions, please jump in.
Do you have the original installation file? If not download it afresh from
http://www.avast.com/eng/download-avast-home.html , you'll need it later.
Download the uninstall utility from
http://www.avast.com/eng/avast-uninstall-utility.html , you'll need that sooner.
It might be worthwhile downloading a good antispyware application, just in case it is malware that has caused the corruption in the Avast and notebook files. (I don't know if it is or not, but it seems possible.) I recommend
http://www.superantispyware.com/download.html .
If you choose to do that, install Superantispyware (other good ones include AVG Antispyware, Asquared, or SpywareTerminator), update it (or one of the others, whatever you choose), update it, and run a full scan. Quarantine anything found. Do not delete- always quarantine, or ignore if known to be a FP.
Disconnect from the internet. Attempt to uninstall Avast from add/remove programs. Chances are you will get the same error message.(If you don't get that message, and it uninstalls cleanly, run the uninstall utility afterwards, anyway.) If so, run the uninstall utility. Reboot when prompted, and follow any other directions it should pop up at you. Reboot at least once following the removal.
Install the fresh download of Avast, and then reconnect to the net. Have it check for updates straight away.
Sorry if this is a bit unclear to follow, feeling a bit tired right now and trying to not stuff up. You might need to read it a couple of times to get the salient points clear.
Let us know how you get on.
