OK, as win2k requires an additional rollup 1 update to be compatible with avast5, so no problem in that regard.
Check the Outlook 2000 > Tools menu > Options > Other > Advanced Options > Add-In Manager and ensure that the avast plug-in is installed and not disabled.
Re "another bug : if I check scan of archive mails, Outlook 2000 crashes."
I wouldn't advise scanning scanning the .pst files not because you are experiencing a crash (why I don't know). If avast finds any infected email in there, trying to remove it could result in the corruption of the .pst file and the inability to access the remainder of the emails in that .pst file.