I put this in Flashget's Options / file manager section:
C:\Program Files\Alwil Software\Avast4\ashQuick.exe
and it works fine for me (hint: make sure your ashquick.exe is in the same path).
also, in the avast settings page, you may want to set avast up with these check-marks, so that you get an avast pop up menu showing virus results after Flashget downloads your file (s).