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var

  Form1: TForm1;

  tSize: int64;

procedure TForm1.GetTotalSize( Path: string; var TotalSize: int64 );

var

  sRec: TSearchRec;

  isFound: boolean;

begin

   isFound := FindFirst( Path + '\*.*', faAnyFile, sRec ) = 0;

   while isFound do

   begin

      if ( sRec.Name <> '.' ) and ( sRec.Name <> '..' ) then

      begin

         if ( sRec.Attr and faDirectory ) = faDirectory then

            GetTotalSize( Path + '\' + sRec.Name, TotalSize );

         TotalSize := TotalSize + sRec.Size;

      end;

      isFound := FindNext( sRec ) = 0;

   end;

   FindClose( sRec );

end;

procedure TForm1.Button1Click(Sender: TObject);

begin

   tSize := 0;

   GetTotalSize( 'C:\WINNT', tSize );

   ShowMessage( IntToStr( tSize ) );

end;