Igor,
many thanks for your response.
I have been using the registry override provided for temporary Avast writes since you announced it with 4.6.623. I have not changed it since then.
I'm sure that you know ... but just to review:
With release 4.6.623 the Webshield did not conform to use of the override and persisted in using the system defined Windows temporary files directory.
With 4.6.635 the Webshield did conform to the use of the registry override. However, it used either the system defined Windows temporary directory or the directory specified in the registry override directly and did not use the _avast_ sub-directory.
It is clear that there were some changes in the temporary file handling between 4.6.639 and 4.6.467.
My system logs confirm that (at least for my system - Win XP Pro SP2) the changes in the handling of Avast temporary files occurred following the system restart required by the installation of Avast 4.6.467. All temporary file writes following the installation are to the _avast_ sub-directory of the system defined Windows temporary directory.
I have reverted my system to 4.6.639 to confirm some information.
In 4.6.639:
1) The Webshlock.txt file was created in the system defined Windows temporary directory or, if the registry override (Avast4TempFolder) was present, in the directory specified by the registry override. The sub-directory _avast_ was not used for the Webshlock.txt file.
2) There were still inconsistencies in the use of the _avast_ sub-directory. This subdirectory was not used by Webshield, its temporary files were written to the directory defined in (1) above.
In 4.6.647:
1) The Webshlock.txt file is created in a sub-directory _avast_ of the system defined Windows temporary directory. If the registry override (Avast4TempFolder) is present it is ignored.
2) The Webshield is now consistent in the use of the _avast_ sub-directory. Its temporary files are now written to the _avast_ sub-directory of the system defined Windows temporary directory.