This is almost certainly a corrupted database problem with 091203-0. Turning off all or part (standard shield, file shield) of avast! will get you operational, but remember it will also disable much of the antivirus capability. You might also think about installing something like the free version of Prevx if you do this, since it is very small and non-intrusive. Others have installed Microsoft Security Essentials, which should actually work pretty well alongside Avast!
