Oceanwaves, THANKS!
This explains it. The problem is, under Win2K, it looks like the built-in user/group names are localized (i.e. instead of "NT-AUTHORITY" the German version of Windows uses "NT-AUTORITÄT"). In Windows XP, this seems to be "fixed" (Windows recognizes the English names as well).
So, it seems that the problem only occurs when ALL of the following conditions are true:
1. you are using Windows 2000
2. you are using non-English version of Windows
3. you installed avast in a folder different from the default (namely, to a folder that only had Everyone/FullControl access right, and nothing else).
Fortunately, the fix seems to be easy for us (not recognize accounts/groups by names).
However, since this seems to be a problem limited to quite a low number of users (see conditions above) I'm not sure if we will release a fix immediately... We'll talk about this.
Thanks
Vlk