The virus is located as an attachment in your Outlook mailbox (“PST archive”).
avast! on-demand scanner doesn’t support actions on the content of PST files (i.e. it’s not able to delete a message or an attachment from there) - only the Outlook plugin, as a part of avast! resident protection, can do that. So, that’s why you get the errors when trying to move it to Chest.
The boot-time scanner doesn’t even unpack PST files, so it doesn’t report any virus.