Actually, avast! can handle the following packers:
ACE, ARC, ARJ, AsPack, BZIP, BZIP2, CAB, DOS executables and Win32 executables (UPX, GZIP archives, LHx, MAPI files (*.pst), MIME, NTFS stream, PEProtect), PEShield, RAR, self-extracting, TAR, UPX, ZIP, ZOO
Hope this help in anyway. Thanks Igor for your good explanation… 8)