This occurs when there is a UNICODE (16bit) -> ANSI (8bit) conversion and some characters are missing in the ANSI table. Such are then replaced by special character, which is often a question mark.
My guess is that you're running on english NT based os, but without Russian (or whatever cyrillic nationality you may be) ANSI codepages installed as default language. Is that the case?
As Windows Exploder is an Unicode app, it has no problem, since no conversion occurs. Avast is ANSI application (it has to work on W9x which support Unicode only poorly), so it may have certain problems with displaying characters which don't fit into default ANSI codepage.