DefTasks.xml - Scanning archive files…
The avast configuration are stored into Windows Registry, avast4.ini, and avast4.mdb (or avast4.xml).
The file deftasks.xml is not a configuration storage. That is, the configuration is not read from it when being used.
Rather, it's a tool that lets us enforce certain settings to your current config. I.e. it is read and parsed every time avast starts (on system boot, usually), and all settings from it are transfered to the actual config storage. The configuration storage is indeed the avast4.mdb in case of Database=ODBC (or avast4.xml in case of Database=XML), in short, avast4.mdb (or .xml) is the file where avast stores its current configuration (besides the ini file).
The reverting-to-the-original-version thing you're seeing is a consequence of the fact that deftasks.xml is normal part of avast installation and therefore the avast auto-repair feature always replaces it to the original version. However, this is not a problem as the import procedure from the modified version is already done at that moment.
On-demand, the user just need to turn on archive scanning.
On-access, avast! by default doesn't look for viruses in packed files. However, it can be enabled. Look at the deftasks.xml file in the avast! folder. All you need to do to enable scanning of archives on-access is to add the line:
<STANDARD--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ</STANDARD--ScanPackers>
after the line
<STANDARD--InvalidEntry>1</STANDARD--InvalidEntry>
Speaking technically, these are file formats (unpacker engines) and not file extensions.
If you're using avast! 4 Professional, the same thing can be done in a more convenient way by editing the resident task, the Packers settings on each provider.
Settings for ashQuick.exe program are available in Professional Edition of avast! and are managed by Enhanced User Interface... If you want to enable archives scanning for ashQuick.exe on your computer, add following lines into your DefTasks.xml file:
<Task>
<DefTaskVersion>1</DefTaskVersion>
<Label>*Quick</Label>
<ScanPackers>All</ScanPackers>
</Task>
Then restart avast! service (if you are using Win9x or ME restart your computer) to reflect made changes...
I looked at the lines further down in the file and the looked similar, but had some extra file-extensions, and "overwrite=no" in the tag, should those additions be added to the tag you suggested too (are they newer additions since your original posting) or....
<OUTLOOK--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</OUTLOOK--ScanPackers>
<MAIL--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</MAIL--ScanPackers>
<P2P--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</P2P--ScanPackers>
<IM--ScanPackers>EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</IM--ScanPackers>
If you add the attribute overwrite="no" you would make the changes unusable:
<OUTLOOK--ScanPackers overwrite="no">EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</OUTLOOK--ScanPackers>
<MAIL--ScanPackers overwrite="no">EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</MAIL--ScanPackers>
<P2P--ScanPackers overwrite="no">EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</P2P--ScanPackers>
<IM--ScanPackers overwrite="no">EXE;ZIP;MIME;RAR;ARJ;TAR;GZ;CAB;ARC;ACE;ZOO;BZIP2;WinExec</IM--ScanPackers>