Restore the whole file, then rerun the scan, and tell me what exactly does it report?
It should be able to move just the virus, without moving the containing dbx file (provided you’re using avast 4.6).

Maybe it would be helpful to post the full path that avast reports (copy it from the virus warning dialog). Isn’t it like

some-path\sent items.dbx\email from: blabla subject: blabla\virus-filename.exe

?