Do you still have old Avast files in the Windows temp folder after the Clean Uninstall? Try to delete them under Windows Safe Mode.

If the problem is somewhere else, then it could be almost “anything”, from your internet cache (which you should delete), your internet connection, or the specific Avast server you are connecting to (and/or MANY other things).

If you are asking about how to perform a “manual update”, as oppose to an automatic update, then right click the Avast tray icon and select to update. This downloads around 200KB, when a new database update is actually available.

You can download the entire database from http://www.avast.com/virus-update-history (not 200KB, but around 30MB, much bigger), but after applying the update, the problem might return tomorrow (or it may not, who knows).

If you can, try the database download, and please report back. If that doesn’t work (you will only know in 12 to 24 hours, after reboot), someone from Avast Team might need to come to this topic and request for some specific logs from you. So, again, please report back to get the help you need.