shai, it depends on what exactly is meant by cleaning. If it's a full removal (including registry associations, startup entries, associated files etc etc) or not.
Also, please note that a vast majority of today's viruses are in fact worms - in which case the infection cannot be "removed" from the files as the files themselves are the infection.
avast features the VRDB which is able to help cleaning many infections, provided it has been generated before the virus hit.
Cheers,
Vlk