I had the same thing, here's what I got from Karel from Alwil:
the mail was originated by one of the last version of the Beagle
worm, the F version or latter (Beagle-J in this case). Those version are
able to send password protected (=encrypted) zip files. The password for
virus decryption is in the mail text. Of course, no virus detection is
possible in the ecrypted files.
After decryption (= un-zipping with the proper password supplied) the
virus is in the executable form and Avast can detect it and prevent
infection of the computer, but Avast cannot spot the virus in the mail
(because of encryption).
--
Regards,
Karel Divis
Virus analyst
Alwil software