Hi,
this is quite normal:
only some TRUE viruses (EXE/COM-file infecting ones) can be repaired.
Trojans and Worms must in most cases be deleted, as the “infected” files contain ONLY copies of the malware
ALWAYS it’s a good idea to inform oneself about the proper desinfection/removal procedure:
see virusinfo on avast, in my sig, or via the boardsearch 