Have you tried a search for it (as it does exist, not in the program folder) using windows explorer. You need to have got rid of the explorer hidden files and folder option.
- Ensure that you have hidden files and folders enabled and disable hide system files in Windows Explorer, Tools, Folder Options, Hidden files and folders, uncheck Hide extensions for known file types, etc. see image.
Though I don't believe it would go in there but the EmailShield.ini file in the same location.
- avast! 5 .ini file locations in Windows XP, C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\EmailShield.ini