procedure TForm1.Button1Click(Sender: TObject);
var
   FreeBytesAvailableToCaller : TLargeInteger;
   FreeSize     : TLargeInteger;
   TotalSize    : TLargeInteger;
begin
     GetDiskFreeSpaceEx('c:',FreeBytesAvailableToCaller,Totalsize,
                         @FreeSize);
end;