[ecfisa]

Hola Lepuke.

Intenta de este modo:

Código Delphi [-]
var
  buf: array[0..50*1024] of Byte;
begin
  with TFileStream.Create('test.exe', fmOpenReadWrite) do
  try
    Seek(0, soFromEnd);
    FillChar(buf, SizeOf(buf), 0);
    Write(buf, SizeOf(buf));
  finally
    Free;
  end;
end;

Saludos