The regular auto update or manual program update for the most part has no problem, I have never had to do a clean install because of a failed update. Occasionally you left with little option but to reinstall and in this case I feel this would be easier than trying to find out why sour system is hanging. If nothing else, a clean install should eliminate avast from the equation if it continues after a clean install.
However, a clean install really isn't a problem, if you have made any changes in the avast4.ini you can always save that file to another folder and when you are done reinstalling copy it back replacing the default one.
I really can't see why you wouldn't want to install from scratch it really isn't that much hassle ?
Download the latest version of avast
http://www.avast.com/eng/programs.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
Now uninstall, reboot, run the uninstall utility, reboot install the latest version, reboot.